autofs: validate protocol version

Move the protocol parameter validation into a seperate function.

Signed-off-by: Ian Kent <raven@themaw.net>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>
Message-Id: <20230922041215.13675-7-raven@themaw.net>
Signed-off-by: Christian Brauner <brauner@kernel.org>