cifs: multichannel: use pointer for binding channel

Add a cifs_chan pointer in struct cifs_ses that points to the channel
currently being bound if ses->binding is true.

Previously it was always the channel past the established count.

This will make reconnecting (and rebinding) a channel easier later on.

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>