x86/pagewalk: Improve the logic behind setting access and dirty bits

x86/pagewalk: Improve the logic behind setting access and dirty bits

The boolean pse2M is misnamed, because it might refer to a 4M superpage.

Switch the logic to be in terms of the level of the leaf entry, and rearrange
the calls to set_ad_bits() to be a fallthrough switch statement, to make it
easier to follow.

Alter set_ad_bits() to take properly typed pointers and booleans rather than
integers.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>