FreeBSD: Fix build on stable/13 after 1302506.

Starting approximately from version 1302506 vn_lock_pair() grown two
additional arguments following head. There is a one week hole, but
that is closet reference point we have.

Reviewed-by: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15047