[NETFILTER]: cleanup nfnetlink_check_attributes()

1) memset return parameter 'cda' (nfattr pointer array) only on success
2) a message without attributes and just a 'struct nfgenmsg' is valid,
   don't return -EINVAL
3) use likely() and unlikely() where apropriate

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>