mm: zcache: fix race between store and evict

The following race is possible:

CPU 1				CPU2
zcache_store_page
zbud_alloc
				zcache_evict_zpage
				zpool = zhandle->zpool;
				CRASH

zcache_store_page
zhandle->zpool = zpool

Fix this by properly initializing the zhandle and validating
in zcache_evict_zpage

Change-Id: I02328220b30f415fa1f171236eab3a2e40072fd9
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>