vm: add kzalloc_node() inline

To get zeroed out memory from a particular NUMA node.  To be used by
sunrpc.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>