lib/waiter: Defer free of removed waiters

lib/waiter: Defer free of removed waiters

We may end up calling remove()-d time waiters if the timeout expires as
we're processing an IO waiter. Instead of freeing the waiter in
waiter_remove, mark the waiter as inactive, and defer the free until the
end of waiter_poll().

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>