mm, page_alloc: allow __GFP_NOFAIL to allocate below watermarks after reclaim

mm, page_alloc: allow __GFP_NOFAIL to allocate below watermarks after reclaim

If direct reclaim has failed to free memory, __GFP_NOFAIL allocations can
potentially loop forever in the page allocator.  In this case, it's better
to give them the ability to access below watermarks so that they may
allocate similar to the same privilege given to GFP_ATOMIC allocations.

We're careful to ensure this is only done after direct reclaim has had the
chance to free memory, however.

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>