lmkd: input valid zone's file cache stats to file_cache_to_adj()

Having both highmem and normal zone in the system, file pages are
preffered to go to highmem and then to normal zone. But in the
file_cache_to_adj() calculations, we first input the normal zone stats
and then fall back to highmem zone. This can cause memory headroom
problem. Rely on the pgskip_(dma|normal|highmem|movable) stats to
decide if to consider particular zone file cache info. If a particular
zone is skipped, then memory pressure is not from allocation request of
that zone thus the file cache info of that zone can be skipped.

CRs-Fixed: 2614374
Change-Id: I2c1e83df66ba9ae20a6c011f83212651635ef01b