x86: Remove invlpg_works_ok and invlpg only single-page regions.

x86: Remove invlpg_works_ok and invlpg only single-page regions.

The flush_area_local() interface was unclear about whether a
multi-page region (2M/4M/1G) had to be mapped by a superpage, and
indeed some callers (map_pages_to_xen()) already would specify
FLUSH_LEVEL(2) for a region actually mapped by 4kB PTEs.

The safest fix is to relax the interface and do a full TLB flush in
these cases. My suspicion is that these cases are rare enough that the
cost of INVLPG versus full flush will be unimportant.

Signed-off-by: Keir Fraser <keir@xensource.com>