mlxsw: spectrum_qdisc: Pass handle, not band number to find_class()

In mlxsw Qdisc offload, find_class() is an operation that yields a qdisc
offload descriptor given a parental qdisc descriptor and a class handle. In
__mlxsw_sp_qdisc_ets_graft() however, a band number is passed to that
function instead of a handle. This can lead to a trigger of a WARN_ON
with the following splat:

 WARNING: CPU: 3 PID: 808 at drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c:1356 __mlxsw_sp_qdisc_ets_graft+0x115/0x130 [mlxsw_spectrum]
 [...]
 Call Trace:
  mlxsw_sp_setup_tc_prio+0xe3/0x100 [mlxsw_spectrum]
  qdisc_offload_graft_helper+0x35/0xa0
  prio_graft+0x176/0x290 [sch_prio]
  qdisc_graft+0xb3/0x540
  tc_modify_qdisc+0x56a/0x8a0
  rtnetlink_rcv_msg+0x12c/0x370
  netlink_rcv_skb+0x49/0xf0
  netlink_unicast+0x1f6/0x2b0
  netlink_sendmsg+0x1fb/0x410
  ____sys_sendmsg+0x1f3/0x220
  ___sys_sendmsg+0x70/0xb0
  __sys_sendmsg+0x54/0xa0
  do_syscall_64+0x3a/0x70
  entry_SYSCALL_64_after_hwframe+0x44/0xae

Since the parent handle is not passed with the offload information, compute
it from the band number and qdisc handle.

Fixes: 28052e618b04 ("mlxsw: spectrum_qdisc: Track children per qdisc")
Reported-by: Maksym Yaremchuk <maksymy@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>