Fix overly broad spa config lock

The spa_txg_history_init_io() and spa_txg_history_fini_io() were
mistakenly taking SCL_ALL when only SCL_CONFIG is required to
access the vdev stats.  This could result in a deadlock which
was observed when running ztest.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8445

module/zfs/vdev.c

@@ -3892,7 +3892,6 @@ vdev_get_stats_ex(vdev_t *vd, vdev_stat_t *vs, vdev_stat_ex_t *vsx)
 			vs->vs_resilver_deferred = vd->vdev_resilver_deferred;
 	}
 
-	ASSERT(spa_config_held(vd->vdev_spa, SCL_ALL, RW_READER) != 0);
 	vdev_get_stats_ex_impl(vd, vs, vsx);
 	mutex_exit(&vd->vdev_stat_lock);
 }