dax: Remove optimisation from dax_lock_mapping_entry

Skipping some of the revalidation after we sleep can lead to returning
a mapping which has already been freed.  Just drop this optimisation.

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>