[NETFILTER]: GRE conntrack: fix htons/htonl confusion

GRE keys are 16 bit.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>