Revert "ip6tnl: fix use after free of fb_tnl_dev"

[ No relevant upstream commit. ]

This reverts commit 22c3ec55.

This patch is not the right fix, it introduces a memory leak when a netns is
destroyed (the FB device is never deleted).

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reported-by: Steven Rostedt <srostedt@redhat.com>
Tested-by: Steven Rostedt <srostedt@redhat.com> (and our entire MRG team)
Tested-by: "Luis Claudio R. Goncalves" <lgoncalv@redhat.com>
Tested-by: John Kacur <jkacur@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>