netstack: Use ctx.BlockWithTimeout() directly while releasing socket.

[copybara-service]

netstack: Use ctx.BlockWithTimeout() directly while releasing socket.

When ctx is of type kernel.taskAsyncContext, calling
kernel.TaskFromContext(ctx) on it returns the original task (even though this
is an async goroutine). And then calling methods like Block() on it would panic
because it expects to be running on the task goroutine. Furthermore, the Task
could have exited so working on a dead task could cause other issues too.

Instead, use kernel.taskAsyncContext methods via the context.Context
interface. This change adds BlockWithTimeout() to the context interface and
implements it for context.NoTask. kernel.Task already implements this.

The current implementation of context.NoTask.BlockWithTimeoutOn was buggy:

• It returned true when the desired event occurred OR when the timeout expired.
  However, context.Blocker interface docstring suggest that true should only
  be returned when the desired event occurs, which kernel.Task implements
  correctly.
• Instead of returning the remaining timeout, it returned the duration passed
  since the timer started.

Fixed these two bugs and made the implementations of kernel.Task and
context.NoTask consistent with one another.

Also clarified docstrings for some Task.Block*() methods about the precondition
of "The caller must be running on the task goroutine.".

Reported-by: [email protected]
Reported-by: [email protected]