bpf: drop useless bpf_fd member from cls/act

After setup we don't need to keep user space fd number around anymore, as
it also has no useful meaning for anyone, just remove it.

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