vdev_ops: don't try to call vdev_op_hold or vdev_op_rele when NULL

This prevents a panic after a SLOG add/removal on the root pool followed
by a zpool scrub.

When a SLOG is removed, a hole takes its place - the vdev_ops for a hole
is vdev_hole_ops, which defines the handler functions of vdev_op_hold
and vdev_op_rele as NULL.