dax: Avoid losing wakeup in dax_lock_mapping_entry

After calling get_unlocked_entry(), you have to call
put_unlocked_entry() to avoid subsequent waiters losing wakeups.

Fixes: c2a7d2a1 ("filesystem-dax: Introduce dax_lock_mapping_entry()")
Cc: stable@vger.kernel.org
Signed-off-by: Matthew Wilcox <willy@infradead.org>

fs/dax.c

@@ -391,6 +391,7 @@ bool dax_lock_mapping_entry(struct page *page)
 			rcu_read_unlock();
 			entry = get_unlocked_entry(&xas);
 			xas_unlock_irq(&xas);
+			put_unlocked_entry(&xas, entry);
 			rcu_read_lock();
 			continue;
 		}