Hold SCL_VDEV when counting leaves

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