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>

fs/dax.c

@@ -383,11 +383,8 @@ bool dax_lock_mapping_entry(struct page *page)
 		entry = xas_load(&xas);
 		if (dax_is_locked(entry)) {
 			entry = get_unlocked_entry(&xas);
-			/* Did the page move while we slept? */
-			if (dax_to_pfn(entry) != page_to_pfn(page)) {
-				xas_unlock_irq(&xas);
-				continue;
-			}
+			xas_unlock_irq(&xas);
+			continue;
 		}
 		dax_lock_entry(&xas, entry);
 		xas_unlock_irq(&xas);