libzfs: const'ify path argument to zfs_path_to_zhandle

zfs_path_to_zhandle has no need to mutate the path argument,
most notably:

• zfs_open takes path as const
• getextmntent takes path as const
• fprintf most clearly doesn't need to mutate it

It's hard to foresee any reason that libzfs could conceivably
want to mutate it in the future, either, so const'ify it.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Kyle Evans <kevans@FreeBSD.org>
Closes #10605