FreeBSD: stop using buffer cache-only routines on sync

Both vop_fsync and vfs_stdsync are effectively just costly no-ops
as they only act on ->v_bufobj.bo_dirty et al, which are unused
by zfs.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Closes #14157