flow_dissector: Drop BPF flow dissector prog ref on netns cleanup

When attaching a flow dissector program to a network namespace with
bpf(BPF_PROG_ATTACH, ...) we grab a reference to bpf_prog.

If netns gets destroyed while a flow dissector is still attached, and there
are no other references to the prog, we leak the reference and the program
remains loaded.

Leak can be reproduced by running flow dissector tests from selftests/bpf:

  # bpftool prog list
  # ./test_flow_dissector.sh
  ...
  selftests: test_flow_dissector [PASS]
  # bpftool prog list
  4: flow_dissector  name _dissect  tag e314084d332a5338  gpl
          loaded_at 2020-05-20T18:50:53+0200  uid 0
          xlated 552B  jited 355B  memlock 4096B  map_ids 3,4
          btf_id 4
  #

Fix it by detaching the flow dissector program when netns is going away.

Fixes: d58e468b ("flow_dissector: implements flow dissector BPF hook")
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20200521083435.560256-1-jakub@cloudflare.com