cifs: do not duplicate fscache cookie for secondary channels

We allocate index cookies for each connection from the client.
However, we don't need this index for each channel in case of
multichannel. So making sure that we avoid creating duplicate
cookies by instantiating only for 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>