Fix overly broad locking in spa_vdev_config_exit()

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.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11585

module/zfs/spa_misc.c

@@ -1274,9 +1274,9 @@ spa_vdev_config_exit(spa_t *spa, vdev_t *vd, uint64_t txg, int error, char *tag)
 		 */
 		vdev_autotrim_stop_wait(vd);
 
-		spa_config_enter(spa, SCL_ALL, spa, RW_WRITER);
+		spa_config_enter(spa, SCL_STATE_ALL, spa, RW_WRITER);
 		vdev_free(vd);
-		spa_config_exit(spa, SCL_ALL, spa);
+		spa_config_exit(spa, SCL_STATE_ALL, spa);
 	}
 
 	/*