x86, pat: avoid highmem cache attribute aliasing

Highmem code can leave ptes and tlb entries around for a given page even after
kunmap, and after it has been freed.

>From what I can gather, the PAT code may change the cache attributes of
arbitrary physical addresses (ie. including highmem pages), which would result
in aliases in the case that it operates on one of these lazy tlb highmem
pages.

Flushing kmaps should solve the problem.

I've also just added code for conditional flushing if we haven't got
any dangling highmem aliases -- this should help performance if we
change page attributes frequently or systems that aren't using much
highmem pages (eg. if < 4G RAM). Should be turned into 2 patches, but
just for RFC...

Signed-off-by: Ingo Molnar <mingo@elte.hu>