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

lib/libzfs/libzfs_util.c

@@ -1104,7 +1104,7 @@ zfs_get_pool_handle(const zfs_handle_t *zhp)
  * fs/vol/snap/bkmark name.
  */
 zfs_handle_t *
-zfs_path_to_zhandle(libzfs_handle_t *hdl, char *path, zfs_type_t argtype)
+zfs_path_to_zhandle(libzfs_handle_t *hdl, const char *path, zfs_type_t argtype)
 {
 	struct stat64 statbuf;
 	struct extmnttab entry;