vxlan: fix byte order of vxlan-gpe port number

vxlan->cfg.dst_port is in network byte order, so an htons()
is needed here. Also reduced comment length to stay closer
to 80 column width (still slightly over, however).

Fixes: e1e5314d ("vxlan: implement GPE")
Signed-off-by: Lance Richardson <lrichard@redhat.com>
Acked-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>