net: sched: gracefully handle qdisc which don't support change()

tc_qdisc_flow_control() incorrectly assumes that all queue disciplines
implement the change() callback in Qdisc_ops. If the flow control function
is called a queue that does not implement change(), we try to jump to a
null function pointer resulting in a panic. The flow control function
does require that the queue discipline support change() in order to
enable/disable the queue. Now we will check if the queue to do flow
control on supports the required funcionality and will WARN_ONCE and
return if the requirements are not met.

[<ffffffc000c5c028>] panic+0x28c
[<ffffffc0000889f4>] die+0x238
[<ffffffc000088a6c>] arm64_notify_die+0x4c
[<ffffffc000088d00>] bad_mode+0xa8
[<ffffffc000ab9ab8>] tc_qdisc_flow_control+0x98
[<ffffffc000c506e8>] rmnet_vnd_ioctl+0x230
[<ffffffc000aac6fc>] dev_ifsioc+0x2a0
[<ffffffc000aacde8>] dev_ioctl+0x644
[<ffffffc000a7db4c>] sock_ioctl+0x4c
[<ffffffc0001af548>] do_vfs_ioctl+0x4b4
[<ffffffc0001af684>] SyS_ioctl+0x60
[<ffffffc0000854b0>] el0_svc_naked+0x24

Change-Id: I85ca324e57814b6c0eac48d2e076e861ab32c260
Signed-off-by: Harout Hedeshian <harouth@codeaurora.org>