[IPV4]: Make __devinet_sysctl_register return an error

Currently, this function is void, so failures in creating
sysctls for new/renamed devices are not reported to anywhere.

Fixing this is another complex (needed?) task, but this
return value is needed during the namespaces creation to
handle the case, when we failed to create "all" and "default"
entries.

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