mm: uninline page_xchg_last_nid()

Andrew Morton pointed out that page_xchg_last_nid() and
reset_page_last_nid() were "getting nuttily large" and asked that it be
investigated.

reset_page_last_nid() is on the page free path and it would be
unfortunate to make that path more expensive than it needs to be.  Due
to the internal use of page_xchg_last_nid() it is already too expensive
but fortunately, it should also be impossible for the page->flags to be
updated in parallel when we call reset_page_last_nid().  Instead of
unlining the function, it uses a simplier implementation that assumes no
parallel updates and should now be sufficiently short for inlining.

page_xchg_last_nid() is called in paths that are already quite expensive
(splitting huge page, fault handling, migration) and it is reasonable to
uninline.  There was not really a good place to place the function but
mm/mmzone.c was the closest fit IMO.

This patch saved 128 bytes of text in the vmlinux file for the kernel
configuration I used for testing automatic NUMA balancing.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>