x86/mm: merge ptwr and mmio_ro page fault handlers

x86/mm: merge ptwr and mmio_ro page fault handlers

Provide a unified entry to avoid going through pte look-up, decode and
emulation cycle more than necessary. The path taken is determined by
the faulting address.

Note that the order of checks is changed in the new function, but the
order of the checks is performed shouldn't matter.

The sole caller is changed to use the new function.

No functional change.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>