net: adjust socket level ICW to cope with ipv6 variant of {recv, send}msg

After the previous patch we have ipv{6,4} variants for {recv,send}msg,
we should use the generic _INET ICW variant to call into the proper
build-in.

This also allows dropping the now unused and rather ugly _INET4 ICW macro

v1 -> v2:
 - use ICW macro to declare inet6_{recv,send}msg
 - fix a couple of checkpatch offender in the code context

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>