xfrm: fix gre key endianess

fl->fl_gre_key is network byte order contrary to fl->fl_icmp_*.
Make xfrm_flowi_{s|d}port return network byte order values for gre
key too.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>