dnotify: Handle errors from fsnotify_add_mark_locked() in fcntl_dirnotify()

fsnotify_add_mark_locked() can fail but we do not check its return
value. This didn't matter before commit 9dd813c1 "fsnotify: Move
mark list head from object into dedicated structure" as none of possible
failures could happen for dnotify but after that commit -ENOMEM can be
returned. Handle this error properly in fcntl_dirnotify() as
otherwise we just hit BUG_ON(dn_mark->dn) in dnotify_free_mark().

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reported-by: syzkaller
Fixes: 9dd813c1
Signed-off-by: Jan Kara <jack@suse.cz>