Replace PAI vtable dispatch with direct calls

The pai_t interface implements C-style polymorphism via function pointers
to abstract over PAC and HPA. This abstraction provides no real benefit:
only two implementations exist, the dispatcher already knows which one to
use, and HPA stubs 2 of 5 operations. Remove the runtime dispatch in
favor of direct calls.

This commit:
- Promotes pac_alloc/expand/shrink/dalloc/time_until_deferred_work to
  external linkage and replaces the pai_t *self parameter with pac_t *pac.
- Promotes hpa_alloc/expand/shrink/dalloc/time_until_deferred_work to
  external linkage and replaces pai_t *self with hpa_shard_t *shard.
- Updates hpa_dalloc_batch's signature to take hpa_shard_t * directly
  and removes the hpa_from_pai container-of helper. Updates internal
  callers in hpa_alloc, hpa_dalloc, and hpa_sec_flush_impl.
- Drops the vtable assignments from pac_init() and hpa_shard_init().
- Replaces pai_alloc/dalloc/etc. dispatch in pa.c with direct calls.
  HPA expand and shrink (which are unconditional failure stubs) are
  skipped entirely for HPA-owned extents.
- Removes the pa_get_pai() helper.
- Updates tests in test/unit/hpa.c and test/unit/hpa_sec_integration.c
  to call hpa_alloc/dalloc/etc. directly.

The pai_t struct field stays as dead weight in pac_t and hpa_shard_t;
it is removed in the next commit along with pai.h itself.

No behavioral changes.

src/pa_extra.c

@@ -17,7 +17,7 @@ pa_shard_prefork0(tsdn_t *tsdn, pa_shard_t *shard) {
 void
 pa_shard_prefork2(tsdn_t *tsdn, pa_shard_t *shard) {
 	if (shard->ever_used_hpa) {
-		hpa_shard_prefork2(tsdn, &shard->hpa_shard);
+		hpa_shard_prefork2(tsdn, &shard->hpa);
 	}
 }
 
@@ -25,7 +25,7 @@ void
 pa_shard_prefork3(tsdn_t *tsdn, pa_shard_t *shard) {
 	malloc_mutex_prefork(tsdn, &shard->pac.grow_mtx);
 	if (shard->ever_used_hpa) {
-		hpa_shard_prefork3(tsdn, &shard->hpa_shard);
+		hpa_shard_prefork3(tsdn, &shard->hpa);
 	}
 }
 
@@ -36,7 +36,7 @@ pa_shard_prefork4(tsdn_t *tsdn, pa_shard_t *shard) {
 	ecache_prefork(tsdn, &shard->pac.ecache_retained);
 	ecache_prefork(tsdn, &shard->pac.ecache_pinned);
 	if (shard->ever_used_hpa) {
-		hpa_shard_prefork4(tsdn, &shard->hpa_shard);
+		hpa_shard_prefork4(tsdn, &shard->hpa);
 	}
 }
 
@@ -56,7 +56,7 @@ pa_shard_postfork_parent(tsdn_t *tsdn, pa_shard_t *shard) {
 	malloc_mutex_postfork_parent(tsdn, &shard->pac.decay_dirty.mtx);
 	malloc_mutex_postfork_parent(tsdn, &shard->pac.decay_muzzy.mtx);
 	if (shard->ever_used_hpa) {
-		hpa_shard_postfork_parent(tsdn, &shard->hpa_shard);
+		hpa_shard_postfork_parent(tsdn, &shard->hpa);
 	}
 }
 
@@ -71,7 +71,7 @@ pa_shard_postfork_child(tsdn_t *tsdn, pa_shard_t *shard) {
 	malloc_mutex_postfork_child(tsdn, &shard->pac.decay_dirty.mtx);
 	malloc_mutex_postfork_child(tsdn, &shard->pac.decay_muzzy.mtx);
 	if (shard->ever_used_hpa) {
-		hpa_shard_postfork_child(tsdn, &shard->hpa_shard);
+		hpa_shard_postfork_child(tsdn, &shard->hpa);
 	}
 }
 
@@ -84,7 +84,7 @@ size_t
 pa_shard_ndirty(const pa_shard_t *shard) {
 	size_t ndirty = ecache_npages_get(&shard->pac.ecache_dirty);
 	if (shard->ever_used_hpa) {
-		ndirty += psset_ndirty(&shard->hpa_shard.psset);
+		ndirty += psset_ndirty(&shard->hpa.psset);
 	}
 	return ndirty;
 }
@@ -177,7 +177,7 @@ pa_shard_stats_merge(tsdn_t *tsdn, pa_shard_t *shard,
 	}
 
 	if (shard->ever_used_hpa) {
-		hpa_shard_stats_merge(tsdn, &shard->hpa_shard, hpa_stats_out);
+		hpa_shard_stats_merge(tsdn, &shard->hpa, hpa_stats_out);
 	}
 }
 
@@ -209,11 +209,11 @@ pa_shard_mtx_stats_read(tsdn_t *tsdn, pa_shard_t *shard,
 
 	if (shard->ever_used_hpa) {
 		pa_shard_mtx_stats_read_single(tsdn, mutex_prof_data,
-		    &shard->hpa_shard.mtx, arena_prof_mutex_hpa_shard);
+		    &shard->hpa.mtx, arena_prof_mutex_hpa_shard);
 		pa_shard_mtx_stats_read_single(tsdn, mutex_prof_data,
-		    &shard->hpa_shard.grow_mtx,
+		    &shard->hpa.grow_mtx,
 		    arena_prof_mutex_hpa_shard_grow);
-		sec_mutex_stats_read(tsdn, &shard->hpa_shard.sec,
+		sec_mutex_stats_read(tsdn, &shard->hpa.sec,
 		    &mutex_prof_data[arena_prof_mutex_hpa_sec]);
 	}
 }