geneve: Remove socket and offload handlers at destruction.

Sockets aren't currently removed from the the global list when
they are destroyed. In addition, offload handlers need to be cleaned
up as well.

Fixes: 0b5e8b8e

 ("net: Add Geneve tunneling protocol driver")
CC: Andy Zhou <azhou@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>