ksm: trivial tidyups

ksm: trivial tidyups

Add NUMA() and DO_NUMA() macros to minimize blight of #ifdef CONFIG_NUMAs
(but indeed we don't want to expand struct rmap_item by nid when not
NUMA).  Add comment, remove "unsigned" from rmap_item->nid, as "int nid"
elsewhere.  Define ksm_merge_across_nodes 1U when #ifndef NUMA to help
optimizing out.  Use ?: in get_kpfn_nid().  Adjust a few comments noticed
in ongoing work.

Leave stable_tree_insert()'s rb_linkage until after the node has been set
up, as unstable_tree_search_insert() does: ksm_thread_mutex and page lock
make either way safe, but we're going to copy and I prefer this precedent.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Petr Holasek <pholasek@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Izik Eidus <izik.eidus@ravellosystems.com>
Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>