When freezing, sometimes we can fail due to a timeout. If this is the case,
sometimes it can be a kernel bug where tasks are deadlocked. It would be
useful to know the kernel stacks to see where tasks are hung if they are.

Note that this is racy: a task might still freeze while we are logging the
various stacks, so we might miss things. But in the case of a deadlock, it
should catch what's wrong.

v2: log all stacks instead of just the unfrozen tasks
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>