libbpf: sync bpf.h

tools/include/uapi/linux/bpf.h got out of sync with actual kernel header.
Update it.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>