mm/slab.c: clean code by removing redundant if condition

mm/slab.c: clean code by removing redundant if condition

The removed code was unnecessary and changed nothing in the flow, since in
case of returning NULL by 'kmem_cache_alloc_node' returning 'freelist'
from the function in question is the same as returning NULL.

Signed-off-by: Mateusz Nosek <mateusznosek0@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Link: https://lkml.kernel.org/r/20200915230329.13002-1-mateusznosek0@gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>