net: flow_dissector: fix RPS on DSA masters

After the blamed patch, __skb_flow_dissect() on the DSA master stopped
adjusting for the length of the DSA headers. This is because it was told
to adjust only if the needed_headroom is zero, aka if there is no DSA
header. Of course, the adjustment should be done only if there _is_ a
DSA header.

Modify the comment too so it is clearer.

Fixes: 4e500251

 ("net: dsa: generalize overhead for taggers that use both headers and trailers")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>