checkpatch: improve the unnecessary OOM message test

Use the context around a patch to avoid missing some candidates.

Link: http://lkml.kernel.org/r/865e874fbae5decc331a849bd8d71c325db6bc80.1496343345.git.joe@perches.com


Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>