bpf: support BPF_PROG_QUERY for BPF_FLOW_DISSECTOR attach_type

target_fd is target namespace. If there is a flow dissector BPF program
attached to that namespace, its (single) id is returned.

v5:
* drop net ref right after rcu unlock (Daniel Borkmann)

v4:
* add missing put_net (Jann Horn)

v3:
* add missing inline to skb_flow_dissector_prog_query static def
  (kbuild test robot <lkp@intel.com>)

v2:
* don't sleep in rcu critical section (Jakub Kicinski)
* check input prog_cnt (exit early)

Cc: Jann Horn <jannh@google.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/syscall.c

@@ -2009,6 +2009,8 @@ static int bpf_prog_query(const union bpf_attr *attr,
 		break;
 	case BPF_LIRC_MODE2:
 		return lirc_prog_query(attr, uattr);
+	case BPF_FLOW_DISSECTOR:
+		return skb_flow_dissector_prog_query(attr, uattr);
 	default:
 		return -EINVAL;
 	}