pass handle to do_unmount()

The same change has already been done for domount(). On macOS platform
we need to have access to zhp to handle devdisks and snapshots.
Also, symmetry is pleasing.

In addition, the code in zpool_disable_datasets which sorts the
mountpoints did not sort the related handle, which meant that the
mountpoint, and the handle that it is paired with, was lost.
You'd get a random handle with the mountpoint.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #12296