mm: clarify __GFP_NOFAIL deprecation status

__GFP_NOFAIL is documented as a deprecated flag since commit
478352e7

 ("mm: add comment about deprecation of __GFP_NOFAIL").

This has discouraged people from using it but in some cases an opencoded
endless loop around allocator has been used instead.  So the allocator
is not aware of the de facto __GFP_NOFAIL allocation because this
information was not communicated properly.

Let's make clear that if the allocation context really cannot afford
failure because there is no good failure policy then using __GFP_NOFAIL
is preferable to opencoding the loop outside of the allocator.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Vipul Pandya <vipul@chelsio.com>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>