datapath: Correct GRE checksum offset.

If a statically defined GRE key is used with checksums then the
checksum will overwrite the key.  This is because the key is set
at header caching time and the checksum is set at packet processing
time and we don't update the offset on packet processing because
we don't do anything with the key.

Reported-by: Christos Argyropoulos <cargious@netmode.ntua.gr>
Signed-off-by: Jesse Gross <jesse@nicira.com>