cachefiles: Handle readpage error correctly

If ->readpage returns an error, it has already unlocked the page.

Fixes: 5e929b3 ("CacheFiles: Handle truncate unlocking the page we're reading")
Cc: stable@vger.kernel.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Matthew Wilcox (Oracle)

fs/cachefiles/rdwr.c

@@ -121,7 +121,7 @@ static int cachefiles_read_reissue(struct cachefiles_object *object,
 		_debug("reissue read");
 		ret = bmapping->a_ops->readpage(NULL, backpage);
 		if (ret < 0)
-			goto unlock_discard;
+			goto discard;
 	}
 
 	/* but the page may have been read before the monitor was installed, so
@@ -138,6 +138,7 @@ static int cachefiles_read_reissue(struct cachefiles_object *object,
 
 unlock_discard:
 	unlock_page(backpage);
+discard:
 	spin_lock_irq(&object->work_lock);
 	list_del(&monitor->op_link);
 	spin_unlock_irq(&object->work_lock);