rmlock: Add a required compiler membar to the rlock slow path

The tracker flags need to be loaded only after the tracker is removed
from its per-CPU queue.  Otherwise, readers may fail to synchronize with
pending writers attempting to propagate priority to active readers, and
readers and writers deadlock on each other.  This was observed in a
stable/12-based armv7 kernel where the compiler had reordered the load
of rmp_flags to before the stores updating the queue.

Reviewed by:	rlibby, scottl
Discussed with:	kib
Sponsored by:	Rubicon Communications, LLC ("Netgate")
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D28821

sys/kern/kern_rmlock.c

@@ -362,7 +362,11 @@ _rm_rlock_hard(struct rmlock *rm, struct rm_priotracker *tracker, int trylock)
 	/* Remove our tracker from the per-cpu list. */
 	rm_tracker_remove(pc, tracker);
 
-	/* Check to see if the IPI granted us the lock after all. */
+	/*
+	 * Check to see if the IPI granted us the lock after all.  The load of
+	 * rmp_flags must happen after the tracker is removed from the list.
+	 */
+	__compiler_membar();
 	if (tracker->rmp_flags) {
 		/* Just add back tracker - we hold the lock. */
 		rm_tracker_add(pc, tracker);