tipc: replace port pointer with socket pointer in registry

In order to make tipc_sock the only entity referencable from other
parts of the stack, we add a tipc_sock pointer instead of a tipc_port
pointer to the registry. As a consequence, we also let the function
tipc_port_lock() return a pointer to a tipc_sock instead  of a tipc_port.
We keep the function's name for now, since the lock still is owned by
the port.

This is another step in the direction of eliminating port_lock, replacing
its usage with lock_sock() and bh_lock_sock().

Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Reviewed-by: Erik Hugne <erik.hugne@ericsson.com>
Reviewed-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>