x86/mm: Rework get_page_and_type_from_mfn conditional

x86/mm: Rework get_page_and_type_from_mfn conditional

Make it easier to read by declaring the conditions in which we will
retain the ref, rather than the conditions under which we release it.

The only way (page == current->arch.old_guest_table) can be true is if
preemptible is true; so remove this from the query itself, and add an
ASSERT() to that effect on the opposite path.

No functional change intended.

NB that alloc_lN_table() mishandle the "linear pt failure" situation
described in the comment; this will be addressed in a future patch.

This is part of XSA-299.

Reported-by: George Dunlap <george.dunlap@citrix.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>