x86/mm: ensure L2 is always freed if empty

x86/mm: ensure L2 is always freed if empty

The current logic in modify_xen_mappings() allows for fully empty L2 tables to
not be freed and unhooked from the parent L3 if the last L2 slot is not
populated.

Ensure that even when an L2 slot is empty the logic to check whether the whole
L2 can be removed is not skipped.

Fixes: 4376c05c3113 ('x86-64: use 1GB pages in 1:1 mapping if available')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>