net/sched: fq_pie: re-factor fix for fq_pie endless loop

commit 3a62fed2fd7b6fea96d720e779cafc30dfb3a22e upstream.

the patch that fixed an endless loop in_fq_pie_init() was not considering
that 65535 is a valid class id. The correct bugfix for this infinite loop
is to change 'idx' to become an u32, like Colin proposed in the past [1].

Fix this as follows:
 - restore 65536 as maximum possible values of 'flows_cnt'
 - use u32 'idx' when iterating on 'q->flows'
 - fix the TDC selftest

This reverts commit bb2f930d6d.

[1] https://lore.kernel.org/netdev/20210407163808.499027-1-colin.king@canonical.com/

CC: Colin Ian King <colin.king@canonical.com>
CC: stable@vger.kernel.org
Fixes: bb2f930d6d ("net/sched: fix infinite loop in sch_fq_pie")
Fixes: ec97ecf1eb ("net: sched: add Flow Queue PIE packet scheduler")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

tools/testing/selftests/tc-testing/tc-tests/qdiscs/fq_pie.json

@@ -9,11 +9,11 @@
         "setup": [
             "$IP link add dev $DUMMY type dummy || /bin/true"
         ],
-        "cmdUnderTest": "$TC qdisc add dev $DUMMY root fq_pie flows 65536",
-        "expExitCode": "2",
+        "cmdUnderTest": "$TC qdisc add dev $DUMMY handle 1: root fq_pie flows 65536",
+        "expExitCode": "0",
         "verifyCmd": "$TC qdisc show dev $DUMMY",
-        "matchPattern": "qdisc",
-        "matchCount": "0",
+        "matchPattern": "qdisc fq_pie 1: root refcnt 2 limit 10240p flows 65536",
+        "matchCount": "1",
         "teardown": [
             "$IP link del dev $DUMMY"
         ]