tunnels: correct conditional build of MPLS and IPv6

Using a combination if #if conditionals and goto labels to unwind
tunnel4_init seems unwieldy. This patch takes a simpler approach of
directly unregistering previously registered protocols when an error
occurs.

This fixes a number of problems with the current implementation
including the potential presence of labels when they are unused
and the potential absence of unregister code when it is needed.

Fixes: 8afe97e5 ("tunnels: support MPLS over IPv4 tunnels")
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>