autofs: add autofs_parse_fd()

Factor out the fd mount option handling.

Signed-off-by: Ian Kent <raven@themaw.net>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>
Message-Id: <20230922041215.13675-3-raven@themaw.net>
Signed-off-by: Christian Brauner <brauner@kernel.org>