iproute2: add support for GRE ignore-df knob

iproute2: add support for GRE ignore-df knob

In the presence of firewalls which improperly block ICMP Unreachable
(including Fragmentation Required) messages, Path MTU Discovery is
prevented from working.

The workaround is to handle IPv4 payloads opaquely, ignoring the DF
bit.

Kernel commit 22a59be8b7693eb2d0897a9638f5991f2f8e4ddd ("net: ipv4:
Add ability to have GRE ignore DF bit in IPv4 payloads") is
complemented by this user-space changeset which exposes control of
this setting.

Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>