qnx6: convert qnx6 to use the new mount api

Convert the qnx6 filesystem to use the new mount API.

Mostly untested, since there is no qnx6 fs image readily available.
Testing did include parsing of the mmi_fs option.

Signed-off-by: Bill O'Donnell <bodonnel@redhat.com>
Link: https://lore.kernel.org/r/20240302165714.859504-1-bodonnel@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>