gre: Use GSO flags to determine csum need instead of GRE flags

This patch updates the gre checksum path to follow something much closer to
the UDP checksum path.  By doing this we can avoid needing to do as much
header inspection and can just make use of the fields we were already
reading in the sk_buff structure.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>