Clean up do_vol_test in zfs_copies tests

Clean up do_vol_test in zfs_copies tests

Get rid of the `get_used_prop` function. `get_prop used` works fine.

Fix the comment describing the function parameters. The type does not
have a default, and mntp is also used for ext2.

Rename the variable for the number of copies from `copy` to `copies`.

Use a `case` statement to match the type parameter, order the cases
alphabetically, and add a little sanity checking for good measure.

Use eval to make sure the output of commands is silenced rather than
the log messages when redirecting output to /dev/null.

Simplify cases where zfs requires special behavior.

Don't allow the test to loop forever in the event space usage does not
change. Bail out of the loop and fail after an arbitrary number of
iterations.

Add more information to the log message when the test fails, to help
debugging.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9286