[NETFILTER]: nfnetlink_log: fix byteorder confusion

flags is a u16, so use htons instead of htonl. Also avoid double
conversion.

Noticed by Alexey Dobriyan <adobriyan@gmail.com>

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>