10701 Correct lock ASSERTs in vdev_label_read/write

illumos/illumos-gate@58447f688d5e308373ab16a3b129bc0ba0fbc154
https://github.com/illumos/illumos-gate/commit/58447f688d5e308373ab16a3b129bc0ba0fbc154

https://www.illumos.org/issues/10701

Port of ZoL commit:
0091d66f4e Correct lock ASSERTs in vdev_label_read/write
At a minimum, this fixes a blown assert during an MMP test run when running on
a DEBUG build.

Portions contributed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Author: Olaf Faaland <faaland1@llnl.gov>