libsa/zfs: ignore labels with zero txg

Zero txg means this vdev has been correctly removed from the pool. For
reference see module/zfs/vdev_removal.c in OpenZFS. It calls
vdev_label_init(vd, 0, VDEV_LABEL_REMOVE), and second argument there is
the txg to write.

Reviewed by: tsoome, mav, imp
Differential Revision: https://reviews.freebsd.org/D51910