FreeBSD: add missing seqc write begin/end around zfs_acl_chown_setattr

It happens to trip over an assert but does not matter for correctness at
this time. Done for future proofing.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Closes #11884