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

module/os/freebsd/zfs/zfs_vfsops.c

@@ -438,10 +438,6 @@ zfs_sync(vfs_t *vfsp, int waitfor)
 		dsl_pool_t *dp;
 		int error;
 
-		error = vfs_stdsync(vfsp, waitfor);
-		if (error != 0)
-			return (error);
-
 		if ((error = zfs_enter(zfsvfs, FTAG)) != 0)
 			return (error);
 		dp = dmu_objset_pool(zfsvfs->z_os);

module/os/freebsd/zfs/zfs_vnops_os.c

@@ -4724,7 +4724,6 @@ static int
 zfs_freebsd_fsync(struct vop_fsync_args *ap)
 {
 
-	vop_stdfsync(ap);
 	return (zfs_fsync(VTOZ(ap->a_vp), 0, ap->a_td->td_ucred));
 }