Fix kselftests build after changes from bz2162116 (ipv4 backports)

Unfortunately the uapi headers changes done through bz2162116 broke kernel builds.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2166738

Any kernel build using kernel-headers-5.14.0-253.el9 will fail when building kselftests.

The reason is that a new macro used in the headers are not included through other header files, upstream doesn't see the error because the definition is included indirectly there.

I identified the commit which fixes this upstream, and made the kselftests build process try to really use the headers from the source tree. Hopefully this prevents us to hit a problem like this in the future. It was also necessary to anyway include <linux/stddef.h> in ip.h/ipv6.h headers, because at least one bpf selftest required it.

Signed-off-by: Herton R. Krzesinski herton@redhat.com