zpool_vdev_name now returns disk# instead of disk#s# for whole-disk vdevs.

We were using Linux's strip_partition function, which is specific to
Linux's naming conventions for devices. Now, to determine the name of
a whole-disk vdev, we strip off s# from the BSD name disk#s#.

Misreporting the vdev name was causing the following issue, and perhaps
other similar issues.

The command

    zpool attach -f tank disk2s1 disk3

would fail if the the user originally created the pool with the command

     zpool create -f tank disk2

even though zpool status was reporting the vdev to be disk2s1 not disk2.

With this fix, zpool status will now report disk2, so that the user knows
the attach command should be

    zpool attach -f tank disk2 disk3

in the case of a whole-disk vdev.

It will continue to be the case that if the create command had been

    zpool create tank disk2s1

then zpool status will report the vdev to be disk2s1, and the user is
expected to use disk2s1 in the attach command.