Correct lock ASSERTs in vdev_label_read/write

Correct lock ASSERTs in vdev_label_read/write

The existing assertions in vdev_label_read() and vdev_label_write(),
testing which config locks are held, are incorrect. The assertions
test for locks which exceed what is required for safety.

Both vdev_label_{read,write}() are changed to assert SCL_STATE is held
as RW_READER or RW_WRITER. This is safe because:

Changes to the vdev tree occur under SCL_ALL as RW_WRITER, via
spa_vdev_enter() and spa_vdev_exit().

Changes to vdev state occur under SCL_STATE_ALL as RW_WRITER, via
spa_vdev_state_enter() and spa_vdev_state_exit().

Therefore, the new assertions guarantee that the vdev cannot change
out from under a zio, and I/O to a specified leaf vdev's label is
safe.

Furthermore, this is consistent with the SPA locking discussion in
spa_misc.c, "For any zio operation that takes an explicit vdev_t
argument ... zio_read_phys(), or zio_write_phys() ... SCL_STATE as
reader suffices."

Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #5983