checkpatch: add warning of future __GFP_NOFAIL use

gfp.h and page_alloc.c already specify that __GFP_NOFAIL is deprecated and
no new users should be added.

Add a warning to checkpatch to catch this.

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