ipx: implement shutdown()

IPX doesn't implement shutdown, which poses a problem to some users:

https://bugzilla.kernel.org/show_bug.cgi?id=67841



This patch is heavily based on the shutdown implementation for unix
sockets.

Reported-by: Bruno Jesus <00cpxxx@gmail.com>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>