mm: Fold page->_last_nid into page->flags where possible

mm: Fold page->_last_nid into page->flags where possible

page->_last_nid fits into page->flags on 64-bit.  The unlikely 32-bit NUMA
configuration with NUMA Balancing will still need an extra page field.  As
Peter notes "Completely dropping 32bit support for CONFIG_NUMA_BALANCING
would simplify things, but it would also remove the warning if we grow
enough 64bit only page-flags to push the last-cpu out."

[mgorman@suse.de: minor modifications]
Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Simon Jeons <simon.jeons@gmail.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>