Fix 'zpool clear' on suspended pools

'zpool clear' should be able to resume I/O on suspended, but otherwise
healthy, pools.

4a283c7 accidentally introduced a new code path where we call
txg_wait_synced() on the suspended pool before we had the chance to
resume I/O via zio_resume(): this results in the 'zpool clear'
command hanging indefinitely, waiting for a TXG that cannot be synced.

Fix this by avoiding the call to txg_wait_synced().

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6399

module/zfs/zfs_ioctl.c

@@ -4946,7 +4946,8 @@ zfs_ioc_clear(zfs_cmd_t *zc)
 
 	vdev_clear(spa, vd);
 
-	(void) spa_vdev_state_exit(spa, spa->spa_root_vdev, 0);
+	(void) spa_vdev_state_exit(spa, spa_suspended(spa) ?
+	    NULL : spa->spa_root_vdev, 0);
 
 	/*
 	 * Resume any suspended I/Os.