waitqueue: add in_waitqueue field

Using the waitqueue node next pointer to figure out if the node is
within the queue worked when wfstack was used to implement the queue
stack, but unfortunately it does not keep the same semantic with
lfstack: indeed, the bottom of stack is indicated by a NULL pointer too.

Keep this state in a separate field instead to eliminate coupling
between workqueue, waitqueue and lfstack.

Note that it is OK for a worker to see "in_waitqueue" as temporarily
false when the wait node has just been pop'd from the waitqueue. It's
the opposite that we never want to happen: a worker should never see
"in_waitqueue" as false when the node is actually in the waitqueue,
because it would cause queue corruption.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>