lowmemorykiller: Don't count swap cache pages twice

The lowmem_shrink function discounts all the swap cache pages from
the file cache count. The zone aware code also discounts all file
cache pages from a certain zone.  This results in some swap cache
pages being discounted twice, which can result in the low memory
killer being unnecessarily aggressive.

Fix the low memory killer to only discount the swap cache pages
once.

Change-Id: I650bbfbf0fbbabd01d82bdb3502b57ff59c3e14f
Signed-off-by: Liam Mark <lmark@codeaurora.org>
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>