[IPV6]: make the protocol initialization to return an error code

This patchset makes the different protocols to return an error code, so
the af_inet6 module can check the initialization was correct or not.

The raw6 was taken into account to be consistent with the rest of the
protocols, but the registration is at the same place.
Because the raw6 has its own init function, the proto and the ops structure
can be moved inside the raw6.c file.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>