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.comSigned-off-by: Christian Brauner <brauner@kernel.org>