Introduce pinned extents to contain unpurgeable pages

Some pages (e.g., hugetlb pages) cannot be purged, and should be
prioritized for reuse.  A custom extent_alloc hook signals this by
OR'ing EXTENT_ALLOC_FLAG_PINNED into the low bits of the returned
pointer; jemalloc strips the flag bits and caches pinned extents in
a dedicated ecache_pinned, separate from the dirty/muzzy decay
pipeline.

Pinned extents do not coalesce eagerly, except for ones larger than
SC_LARGE_MINCLASS.  A prefer-small policy reuses the smallest fitting
pinned extent, to avoid unnecessary split/fragmentation.

include/jemalloc/internal/ctl.h

@@ -63,6 +63,7 @@ typedef struct ctl_stats_s {
 	size_t resident;
 	size_t mapped;
 	size_t retained;
+	size_t pinned;
 
 	background_thread_stats_t background_thread;
 	mutex_prof_data_t mutex_prof_data[mutex_prof_num_global_mutexes];