mm: add comment about deprecation of __GFP_NOFAIL

__GFP_NOFAIL was deprecated in dab48dab, so add a comment that no new
users should be added.

Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.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>