x86/mm: account for PGT_pae_xen_l2 in recently added assertion

x86/mm: account for PGT_pae_xen_l2 in recently added assertion

While PGT_pae_xen_l2 will be zapped once the type refcount of an L2 page
reaches zero, it'll be retained as long as the type refcount is non-
zero. Hence any checking against the requested type needs to either zap
the bit from the type or include it in the used mask.

Fixes: 9186e96b199e ("x86/pv: Clean up _get_page_type()")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>