Fix broken backport for IPv6 tunnels in 2.6.32-longterm kernels.

upstream commit d5aa407f ("tunnels: fix
netns vs proto registration ordering") , which was included in
2.6.32.44-longterm, was not backported correctly, and results in a NULL
pointer dereference in ip6_tunnel.c for longterm kernels >=2.6.32.44

Use [un]register_pernet_gen_device() instead of
[un]register_pernet_device() to fix it.
Signed-off-by: Stratos Psomadakis <psomas@gentoo.org>
Cc: Wolfgang Walter <wolfgang.walter@stwm.de>
Cc: Tim Gardner <tim.gardner@canonical.com>
Cc: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>