mm/mempool: use might_alloc()

mempool are generally used for GFP_NOIO, so this wont benefit all that
much because might_alloc currently only checks GFP_NOFS.  But it does
validate against mmu notifier pte zapping, some might catch some drivers
doing really silly things, plus it's a bit more meaningful in what we're
checking for here.

Link: https://lkml.kernel.org/r/20220605152539.3196045-3-daniel.vetter@ffwll.ch


Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>