x86/hvm: Make the altp2m locking in hvm_hap_nested_page_fault() easier to follow

x86/hvm: Make the altp2m locking in hvm_hap_nested_page_fault() easier to follow

Drop the ap2m_active boolean, and consistently use the unlocking form:

  if ( p2m != hostp2m )
       __put_gfn(p2m, gfn);
  __put_gfn(hostp2m, gfn);

which makes it clear that we always unlock the altp2m's gfn if it is in use,
and always unlock the hostp2m's gfn.  This also drops the ternary expression
in the logdirty case.

Extend the logdirty comment to identify where the locking violation is liable
to occur.

No (intended) overall change in behaviour.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>