mm, page_alloc: warn_alloc nodemask is NULL when cpusets are disabled

The patch "mm, page_alloc: warn_alloc print nodemask" implicitly sets
the allocation nodemask to cpuset_current_mems_allowed when there is no
effective mempolicy.  cpuset_current_mems_allowed is only effective when
cpusets are enabled, which is also printed by warn_alloc(), so setting
the nodemask to cpuset_current_mems_allowed is redundant and prevents
debugging issues where ac->nodemask is not set properly in the page
allocator.

This provides better debugging output since
cpuset_print_current_mems_allowed() is already provided.

Link: http://lkml.kernel.org/r/alpine.DEB.2.10.1701181347320.142399@chino.kir.corp.google.com


Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>