gre: Don't require incoming checksum.

gre: Don't require incoming checksum.

The current meaning of the GRE checksum option is to include
checksums on transmit and require packets to have them on receive.
In addition, incoming packets with checksums are always validated
regardless of this option.  Requiring checksums on receive creates
surprising behavior and interoperability issues.  This disables the
requirement on receive.  The new behavior is that the sender decides
whether to checksum packets and the receiver will validate packets
with checksums (similar to UDP).

Signed-off-by: Jesse Gross <jesse@nicira.com>