mips: fix bitops

Documentation/atomic_ops.txt defines these primitives must contain a memory
barrier both before and after their memory operation.  This is consistent with
the atomic ops implementation on mips.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>