mm/zswap: change zswap to writethrough cache

Currently, zswap is writeback cache; stored pages are not sent
to swap disk, and when zswap wants to evict old pages it must
first write them back to swap cache/disk manually.  This avoids
swap out disk I/O up front, but only moves that disk I/O to
the writeback case (for pages that are evicted), and adds the
overhead of having to uncompress the evicted pages, and adds the
need for an additional free page (to store the uncompressed page)
at a time of likely high memory pressure.  Additionally, being
writeback adds complexity to zswap by having to perform the
writeback on page eviction.

This changes zswap to writethrough cache by enabling
frontswap_writethrough() before registering, so that any
successful page store will also be written to swap disk.  All the
writeback code is removed since it is no longer needed, and the
only operation during a page eviction is now to remove the entry
from the tree and free it.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>