x86/mm: skip super-page alignment checks for non-present entries

x86/mm: skip super-page alignment checks for non-present entries

INVALID_MFN is ~0, so by it having all bits as 1s it doesn't fulfill the
super-page address alignment checks for L3 and L2 entries.  Skip the alignment
checks if the new entry is a non-present one.

This fixes a regression introduced by 0b6b51a69f4d, where the switch from 0 to
INVALID_MFN caused all super-pages to be shattered when attempting to remove
mappings by passing INVALID_MFN instead of 0.

Fixes: 0b6b51a69f4d ('xen/mm: Switch map_pages_to_xen to use MFN typesafe')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>