sch_qfq: prevent shift-out-of-bounds in qfq_init_qdisc

commit 7d18a078 upstream.

tx_queue_len can be set to ~0U, we need to be more
careful about overflows.

__fls(0) is undefined, as this report shows:

UBSAN: shift-out-of-bounds in net/sched/sch_qfq.c:1430:24
shift exponent 51770272 is too large for 32-bit type 'int'
CPU: 0 PID: 25574 Comm: syz-executor.0 Not tainted 5.16.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0x201/0x2d8 lib/dump_stack.c:106
 ubsan_epilogue lib/ubsan.c:151 [inline]
 __ubsan_handle_shift_out_of_bounds+0x494/0x530 lib/ubsan.c:330
 qfq_init_qdisc+0x43f/0x450 net/sched/sch_qfq.c:1430
 qdisc_create+0x895/0x1430 net/sched/sch_api.c:1253
 tc_modify_qdisc+0x9d9/0x1e20 net/sched/sch_api.c:1660
 rtnetlink_rcv_msg+0x934/0xe60 net/core/rtnetlink.c:5571
 netlink_rcv_skb+0x200/0x470 net/netlink/af_netlink.c:2496
 netlink...