this command hangs forever:

 # tc qdisc add dev eth0 root fq_pie flows 65536

 watchdog: BUG: soft lockup - CPU#1 stuck for 23s! [tc:1028]
 [...]
 CPU: 1 PID: 1028 Comm: tc Not tainted 5.7.0-rc6+ #167
 RIP: 0010:fq_pie_init+0x60e/0x8b7 [sch_fq_pie]
 Code: 4c 89 65 50 48 89 f8 48 c1 e8 03 42 80 3c 30 00 0f 85 2a 02 00 00 48 8d 7d 10 4c 89 65 58 48 89 f8 48 c1 e8 03 42 80 3c 30 00 <0f> 85 a7 01 00 00 48 8d 7d 18 48 c7 45 10 46 c3 23 00 48 89 f8 48
 RSP: 0018:ffff888138d67468 EFLAGS: 00000246 ORIG_RAX: ffffffffffffff13
 RAX: 1ffff9200018d2b2 RBX: ffff888139c1c400 RCX: ffffffffffffffff
 RDX: 000000000000c5e8 RSI: ffffc900000e5000 RDI: ffffc90000c69590
 RBP: ffffc90000c69580 R08: fffffbfff79a9699 R09: fffffbfff79a9699
 R10: 0000000000000700 R11: fffffbfff79a9698 R12: ffffc90000c695d0
 R13: 0000000000000000 R14: dffffc0000000000 R15: 000000002347c5e8
 FS:  00007f01e1850e40(0000) GS:ffff88814c880000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 000000000067c340 CR3: 000000013864c000 CR4: 0000000000340ee0
 Call Trace:
  qdisc_create+0x3fd/0xeb0
  tc_modify_qdisc+0x3be/0x14a0
  rtnetlink_rcv_msg+0x5f3/0x920
  netlink_rcv_skb+0x121/0x350
  netlink_unicast+0x439/0x630
  netlink_sendmsg+0x714/0xbf0
  sock_sendmsg+0xe2/0x110
  ____sys_sendmsg+0x5b4/0x890
  ___sys_sendmsg+0xe9/0x160
  __sys_sendmsg+0xd3/0x170
  do_syscall_64+0x9a/0x370
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

we can't accept 65536 as a valid number for 'nflows', because the loop on
'idx' in fq_pie_init() will never end. The extack message is correct, but
it doesn't say that 0 is not a valid number for 'flows': while at it, fix
this also. Add a tdc selftest to check correct validation of 'flows'.

CC: Ivan Vecera <ivecera@redhat.com>
Fixes: ec97ecf1 ("net: sched: add Flow Queue PIE packet scheduler")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Reviewed-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>