ggated(8): Avoid doubly opening the requested disk device.

• Initialize the disk device fd field in connection_new().
• Close the disk device after handing the connection over to a child worker.
• Avoid re-opening a disk device for each connection from the same client, avoiding an fd leak.

PR: 132845
Submitted by: Yoshihiro Ota <ota@j.email.ne.jp>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D26168