Bluetooth: 6lowpan: fix use after free in chan_suspend/resume

A status field in the skb_cb struct was storing a channel status
based on channel suspend/resume events.  This stored status was
then used to return EAGAIN if there were packet sending issues
in snd_pkt().

The issue is that the skb has been freed by the time the callback
to 6lowpan's suspend/resume was called.  So, this generates a
"use after free" issue that was noticed while running kernel tests
with KASAN debug enabled.

Let's eliminate the status field entirely as we can use the channel
tx_credits to indicate whether we should return EAGAIN when handling
packets.

Signed-off-by: Michael Scott <michael.scott@linaro.org>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>