cifs: handle cases where multiple sessions share connection

Based on our implementation of multichannel, it is entirely
possible that a server struct may not be found in any channel
of an SMB session.

In such cases, we should be prepared to move on and search for
the server struct in the next session.

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