cifs: fix leak of iface for primary channel

My last change in this area introduced a change which
accounted for primary channel in the interface ref count.
However, it did not reduce this ref count on deallocation
of the primary channel. i.e. during umount.

Fixing this leak here, by dropping this ref count for
primary channel while freeing up the session.

Fixes: fa1d0508 ("cifs: account for primary channel in the interface list")
Cc: stable@vger.kernel.org
Reported-by: Paulo Alcantara <pc@manguebit.com>
Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>