mm: batch-free pcp list if possible

free_pcppages_bulk() frees pages from pcp lists in a round-robin fashion
by keeping batch_free counter.  But it doesn't need to spin if there is
only one non-empty list.  This can be checked by batch_free ==
MIGRATE_PCPTYPES.

[akpm@linux-foundation.org: fix comment]
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>