Revert "rtnetlink: return EINVAL when request cannot succeed"

This reverts commit b6177d32



ip-link command is testing kernel capability by sending a RTM_NEWLINK
request, without any argument. It accepts everything in reply, except
EOPNOTSUPP and EINVAL (functions iplink_have_newlink / accept_msg)

So we must keep compatiblity here, invalid empty message should not
return EINVAL

Signed-off-by: Florent Fourcot <florent.fourcot@wifirst.fr>
Tested-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>