cifs: always iterate smb sessions using primary channel

smb sessions and tcons currently hang off primary channel only.
Secondary channels have the lists as empty. Whenever there's a
need to iterate sessions or tcons, we should use the list in the
corresponding primary channel.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>