zfs: fix overly broad locking in spa_vdev_config_exit()
Resolves a deadlock which can occur when the ZED or zpool
command attaches a new device.
From the openzfs 75a089ed3 commit message:
Calling vdev_free() only requires the we acquire the spa config SCL_STATE_ALL locks, not the SCL_ALL locks. In particular, we need need to avoid taking the SCL_CONFIG lock (included in SCL_ALL) as a writer since this can lead to a deadlock. The txg_sync_thread() may block in spa_txg_history_init_io() when taking the SCL_CONFIG lock as a reading when it detects there's a pending writer.
Obtained from: openzfs/zfs@75a089ed34befb8d11df9be92a4438cc49554bf6
Approved by: re (delphij)
(cherry picked from commit f08e17c8a251210b58d63b52bfe1a77dccff6e78)