Pages which are under writeout to swap are locked, and not
PageWriteback().  So page allocators do not throttle against them in
shrink_caches().

This causes enormous list scans and general coma under really heavy
swapout loads.

One fix would be to teach shrink_cache() to wait on PG_locked for swap
pages.  The other approach is to set both PG_locked and PG_writeback
for swap pages so they can be handled in the same manner as file-backed
pages in shrink_cache().

This patch takes the latter approach.