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 <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Olaf Faaland <[email protected]>
Closes #5983 

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