cifs: pick channel for tcon and tdis

Today, the tree connect and disconnect requests are
sent on the primary channel only. However, the new
multichannel logic allows the session to remain active
even if one of the channels are alive. So a tree connect
can now be triggered during a reconnect on any of
its channels.

This change changes tcon and tdis calls to pick an
active channel instead of the first one.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>