cifs: generate signkey for the channel that's reconnecting

Before my changes to how multichannel reconnects work, the
primary channel was always used to do a non-binding session
setup. With my changes, that is not the case anymore.
Missed this place where channel at index 0 was forcibly
updated with the signing key.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Cc: stable@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>