mm: zcache: fix accouting of pool pages

zcache_pool_pages is supposed to store the total
pages in all the pools used by zcache. But at
present zcache_pool_pages is assigned pages of any
particular pool on a load, store or any operation
which modifies the pool size. And this is more
important for external clients which depend on zcache
pool size, like the lowmemorykiller.

Change-Id: Ifdaab8646c40f1fec71dfa5903658fbdc6b3cce5
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>