vti6: Don't unregister pernet ops twice on init errors

If we fail to register one of the xfrm protocol handlers we will
unregister the pernet ops twice on the error exit path. This will
probably lead to a kernel panic as the double deregistration
leads to a double kfree().

Fix this by removing one of the calls to do it only once.

Fixes: fa9ad96d49 ("vti6: Update the ipv6 side to use its own...")
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>