dax: Fix dax_unlock_mapping_entry for PMD pages

Device DAX PMD pages do not set the PageHead bit for compound pages.
Fix for now by retrieving the PMD bit from the entry, but eventually we
will be passed the page size by the caller.

Reported-by: Dan Williams <dan.j.williams@intel.com>
Fixes: 9f32d221 ("dax: Convert dax_lock_mapping_entry to XArray")
Signed-off-by: Matthew Wilcox <willy@infradead.org>