ipv6: In fragmentation code, handle error returned from register_pernet_subsys.

The error code is ignored now, but ipv6 is a module and one can
be loaded under memory pressure, so the error may occur (in theory).

Besides, I'm going to handle error returned from registering a
read-only part of the table, so ignoring this one, while handing
the other one would look strange.

(However, this possibility of error is rather small, so I'm not 
 sure whether this is a candidate for current net tree).

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>