Fix the time-since computation in HPA.

nstime module guarantees monotonic clock update within a single nstime_t.  This
means, if two separate nstime_t variables are read and updated separately,
nstime_subtract between them may result in underflow.  Fixed by switching to the
time since utility provided by nstime.

include/jemalloc/internal/hpa_hooks.h

@@ -9,6 +9,7 @@ struct hpa_hooks_s {
 	void (*hugify)(void *ptr, size_t size);
 	void (*dehugify)(void *ptr, size_t size);
 	void (*curtime)(nstime_t *r_time, bool first_reading);
+	uint64_t (*ms_since)(nstime_t *r_time);
 };
 
 extern hpa_hooks_t hpa_hooks_default;