net: genetlink: return the error code when attribute parsing fails.

Currently if attribute parsing fails and the genl family
does not support parallel operation, the error code returned
by __nlmsg_parse() is discarded by genl_family_rcv_msg_attrs_parse().

Be sure to report the error for all genl families.

Fixes: c10e6cf8 ("net: genetlink: push attrbuf allocation and parsing to a separate function")
Fixes: ab5b526d

 ("net: genetlink: always allocate separate attrs for dumpit ops")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>