lib/stackdepot.c: use __GFP_NOWARN for stack allocations

lib/stackdepot.c: use __GFP_NOWARN for stack allocations

This (large, atomic) allocation attempt can fail.  We expect and handle
that, so avoid the scary warning.

Link: http://lkml.kernel.org/r/20160720151905.GB19146@node.shutemov.name
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Rik van Riel <riel@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>