A config lock should be held while vdev_count_leaves() walks the tree,
otherwise the pointers reference may become invalid during the walk.
SCL_VDEV is a minimal lock provided for such uses cases.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes#7286
743253df70