[NETFILTER]: nfnetlink: remove duplicate checks in nfnetlink_check_attributes

Remove nfnetlink_check_attributes duplicates message size and callback
id checks. nfnetlink_find_client and nfnetlink_rcv_msg already do
such checks.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>