Since vxlan is new-ish, and vxlan IPv6 support in particular has only
been in the kernel since 3.11, we include our own copy of the vxlan
netlink constants rather than depending on the installed headers.
This requires a very recent kernel to even compile, and the kernel
code is still rapidly changing (eg, adding IPv6 support). So take it
out for now, until it stabilizes.
This reverts commit 7f0f04d106.