tipc: eliminate function tipc_port_shutdown()

tipc_port_shutdown() is a remnant from the now obsolete native
interface. As such it grabs port_lock in order to protect itself
from concurrent BH processing.

However, after the recent changes to the port/socket upcalls, sockets
are now basically single-threaded, and all execution, except the read-only
tipc_sk_timer(), is executing within the protection of lock_sock(). So
the use of port_lock is not needed here.

In this commit we eliminate the whole function, and merge it into its
only caller, tipc_shutdown().

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>