locks: pass correct "before" pointer to locks_unlink_lock in generic_add_lease

The argument to locks_unlink_lock can't be just any pointer to a
pointer. It must be a pointer to the fl_next field in the previous
lock in the list.

Cc: <stable@vger.kernel.org> # v3.15+
Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>