Fix kstat state update during pool transition

When reading kstats, the health (aka state) of the pool is stored into
/proc/spl/kstat/zfs/POOLNAME/state via spa_state_to_name().
However, during import/export there is a case where the spa exists,
but the root vdev does not exist. This fix checks that case and sets
the state to "TRANSITIONING"

Unfortunately, it is not easy to reproduce a test for this. It was
detected randomly during ZTS runs while kstats were also being sampled
regularly. After this change, further testing did not trip on the case
and the TRANSITIONING state was collected at least once by the kstats.

For posterity, the backtrace prior to this fix is:
[Mon May 13 17:21:00 2019] RIP: 0010:spa_state_to_name+0x10/0xb0 [zfs]
...
Mon May 13 17:21:00 2019] Call Trace:
[Mon May 13 17:21:00 2019]  spa_state_data+0x1a/0x40 [zfs]
[Mon May 13 17:21:00 2019]  kstat_seq_show+0x117/0x440 [spl]
[Mon May 13 17:21:00 2019]  seq_read+0xe5/0x430
[Mon May 13 17:21:00 2019]  proc_reg_read+0x45/0x70
[Mon May 13 17:21:00 2019]  __vfs_read+0x1b/0x40
[Mon May 13 17:21:00 2019]  vfs_read+0x8e/0x130
[Mon May 13 17:21:00 2019]  SyS_read+0x55/0xc0
[Mon May 13 17:21:00 2019]  ? SyS_fcntl+0x5d/0xb0
[Mon May 13 17:21:00 2019]  do_syscall_64+0x73/0x130
[Mon May 13 17:21:00 2019]  entry_SYSCALL_64_after_hwframe+0x3d/0xa2

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>
Signed-off-by: Richard Elling <[email protected]>
Closes #8746

module/zfs/spa_misc.c

@@ -2582,8 +2582,18 @@ spa_set_missing_tvds(spa_t *spa, uint64_t missing)
 const char *
 spa_state_to_name(spa_t *spa)
 {
-	vdev_state_t state = spa->spa_root_vdev->vdev_state;
-	vdev_aux_t aux = spa->spa_root_vdev->vdev_stat.vs_aux;
+	ASSERT3P(spa, !=, NULL);
+
+	/*
+	 * it is possible for the spa to exist, without root vdev
+	 * as the spa transitions during import/export
+	 */
+	vdev_t *rvd = spa->spa_root_vdev;
+	if (rvd == NULL) {
+		return ("TRANSITIONING");
+	}
+	vdev_state_t state = rvd->vdev_state;
+	vdev_aux_t aux = rvd->vdev_stat.vs_aux;
 
 	if (spa_suspended(spa) &&
 	    (spa_get_failmode(spa) != ZIO_FAILURE_MODE_CONTINUE))