geneve: ensure ECN info is handled properly in all tx/rx paths

Partially due to a pre-exising "thinko", the new metadata-based tx/rx
paths were handling ECN propagation differently than the traditional
tx/rx paths.  This patch removes the "thinko" (involving multiple
ip_hdr assignments) on the rx path and corrects the ECN handling on
both the rx and tx paths.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Reviewed-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>