tunnels: Enable UDP checksum computation for Geneve and VXLAN.

tunnels: Enable UDP checksum computation for Geneve and VXLAN.

The kernel module can already support outer UDP checksums for
Geneve and VXLAN using the standard checksum flag in tunnel
metadata. This makes userspace aware of the capability so that
users can enable it on tunnel ports.

There is a complication in that there is no way for userspace to
probe or detect if the kernel does not support this capability
in order to warn the user. In this case, connectivity will appear
to function normally but packets will not be checksum protected.
This is mainly an issue for VXLAN which has existed in the kernel
for a some time without checksum support - while there are also
a few kernel versions that support Geneve only without checksums,
they are much less common.

There isn't a particularly good solution to the compatibility
issue without introducing a larger capabilities structure. However,
UDP checksums are likely to be used only rarely at this point in
time and the VXLAN spec (where the main problem lies) recommends
against them. Therefore, this is considered to be an advanced user
feature and we settle for just documenting the issue.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Pritesh Kothari <pritesh.kothari@cisco.com>