slub: remove slab_alloc wrapper

Being slab_alloc such a simple and unconditional wrapper around
slab_alloc_node, we should get rid of it for simplicity, patching
the callers directly.

Signed-off-by: Glauber Costa <glommer@parallels.com>
CC: Christoph Lameter <cl@linux.com>
CC: David Rientjes <rientjes@google.com>
CC: Pekka Enberg <penberg@cs.helsinki.fi>
CC: Andrew Morton <akpm@linux-foundation.org>