bpf, dst: add and use dst_tclassid helper

We can just add a small helper dst_tclassid() for retrieving the
dst->tclassid value. It makes the code a bit better in that we can
get rid of the ifdef from filter.c by moving this into the header.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>