mm: zcache: add evict zpages supporting

Implemented zbud_ops->evict, so that compressed zpages can be evicted from
zbud memory pool in the case that the compressed pool is full.

zbud already managered the compressed pool based on LRU. The evict was
implemented just by dropping the compressed file page data directly, if
the data is required again then no more disk reading can be saved.

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Patch-mainline: linux-mm @ 2013-08-06 11:36:16
[vinmenon@codeaurora.org: trivial merge conflict fixes]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
Change-Id: Ia66652475e490f0233547511e80abf7587054e65