[TIPC]: Add argument validation for shutdown()

This patch validates that the "how" argument to shutdown()
is SHUT_RDWR, since this is the only form that TIPC supports.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc net/tipc/socket.c
Simple merge