slub: Add might_sleep_if() to slab_alloc()

Currently SLUB doesn't warn about __GFP_WAIT. Add it into slab_alloc().
Acked-by: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>

mm/slub.c

@@ -1591,6 +1591,7 @@ static __always_inline void *slab_alloc(struct kmem_cache *s,
 	unsigned long flags;
 	unsigned int objsize;
 
+	might_sleep_if(gfpflags & __GFP_WAIT);
 	local_irq_save(flags);
 	c = get_cpu_slab(s, smp_processor_id());
 	objsize = c->objsize;