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."

[[email protected]: minor modifications]
Signed-off-by: Mel Gorman <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Simon Jeons <[email protected]>
Cc: Wanpeng Li <[email protected]>
Cc: Hugh Dickins <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>