mm: numa: limit scope of lock for NUMA migrate rate limiting

mm: numa: limit scope of lock for NUMA migrate rate limiting

NUMA migrate rate limiting protects a migration counter and window using a
lock but in some cases this can be a contended lock.  It is not critical
that the number of pages be perfect, lost updates are acceptable.  Reduce
the importance of this lock.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Alex Thorlton <athorlton@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>