checkpatch: make the memory barrier test noisier

Peter Zijlstra prefers that comments be required near uses of memory
barriers.

Change the message level for memory barrier uses from a --strict test only
to a normal WARN so it's always emitted.

This might produce false positives around insertions of memory barriers
when a comment is outside the patch context block.

And checkpatch is still stupid, it only looks for existence of any
comment, not at the comment content.

Signed-off-by: Joe Perches <joe@perches.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>