Move arena_t's purging field into arena_decay_t.

include/jemalloc/internal/arena_structs_b.h

@@ -39,6 +39,11 @@ struct arena_bin_info_s {
 struct arena_decay_s {
 	/* Synchronizes all non-atomic fields. */
 	malloc_mutex_t		mtx;
+	/*
+	 * True if a thread is currently purging the extents associated with
+	 * this decay structure.
+	 */
+	bool			purging;
 	/*
 	 * Approximate time in seconds from the creation of a set of unused
 	 * dirty pages until an equivalent set of unused dirty pages is purged
@@ -199,13 +204,6 @@ struct arena_s {
 	 */
 	arena_decay_t		decay;
 
-	/*
-	 * True if a thread is currently executing arena_purge_to_limit().
-	 *
-	 * Synchronization: decay.mtx.
-	 */
-	bool			purging;
-
 	/*
 	 * Next extent size class in a growing series to use when satisfying a
 	 * request via the extent hooks (only if !config_munmap).  This limits