oom: do not take callback_mutex

Since no task descriptor's 'cpuset' field is dereferenced in the execution of
the OOM killer anymore, it is no longer necessary to take callback_mutex.

[akpm@linux-foundation.org: restore cpuset_lock for other patches]
Cc: Andrea Arcangeli <andrea@suse.de>
Acked-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -463,7 +463,6 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask, int order)
 	 * NUMA) that may require different handling.
 	 */
 	constraint = constrained_alloc(zonelist, gfp_mask);
-	cpuset_lock();
 	read_lock(&tasklist_lock);
 
 	switch (constraint) {
@@ -495,7 +494,6 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask, int order)
 		/* Found nothing?!?! Either we hang forever, or we panic. */
 		if (!p) {
 			read_unlock(&tasklist_lock);
-			cpuset_unlock();
 			panic("Out of memory and no killable processes...\n");
 		}
 
@@ -508,7 +506,6 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask, int order)
 
 out:
 	read_unlock(&tasklist_lock);
-	cpuset_unlock();
 
 	/*
 	 * Give "p" a good chance of killing itself before we