autofs: fix protocol sub version setting

There were a number of updates to protocol version 4, take account of
that when setting the super block info sub version field.

Signed-off-by: Ian Kent <raven@themaw.net>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>
Message-Id: <20230922041215.13675-9-raven@themaw.net>
Signed-off-by: Christian Brauner <brauner@kernel.org>