Fix vdev_rebuild_thread deadlock

The metaslab_disable() call may block waiting for a txg sync.
Therefore it's important that vdev_rebuild_thread release the
SCL_CONFIG read lock it is holding before this call.  Failure
to do so can result in the txg_sync thread getting blocked
waiting for this lock which results in a deadlock.
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Reviewd-by: Srikanth N S <srikanth.nagasubbaraoseetharaman@hpe.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11647 

module/zfs/vdev_rebuild.c

@@ -807,8 +807,8 @@ vdev_rebuild_thread(void *arg)
 		ASSERT0(range_tree_space(vr->vr_scan_tree));
 
 		/* Disable any new allocations to this metaslab */
-		metaslab_disable(msp);
 		spa_config_exit(spa, SCL_CONFIG, FTAG);
+		metaslab_disable(msp);
 
 		mutex_enter(&msp->ms_sync_lock);
 		mutex_enter(&msp->ms_lock);