bpf: xdp: Register generic_kfunc_set with XDP programs

Registering generic_kfunc_set with XDP programs enables some of the
newer BPF features inside XDP -- namely tree based data structures and
BPF exceptions.

The current motivation for this commit is to enable assertions inside
XDP bpf progs. Assertions are a standard and useful tool to encode
intent.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/d07d4614b81ca6aada44fcb89bb6b618fb66e4ca.1702594357.git.dxu@dxuuu.xyz


Signed-off-by: Alexei Starovoitov <ast@kernel.org>