zfs: fix overly broad locking in spa_vdev_config_exit()

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)