The shrink_zone() logic can, under some circumstances, cause far too many
pages to be reclaimed.  Say, we're scanning at high priority and suddenly hit
a large number of reclaimable pages on the LRU.

Change things so we bale out when SWAP_CLUSTER_MAX pages have been reclaimed.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>