pipe: wakeup wr_wait after setting max_usage

In the Linux kernel, the following vulnerability has been resolved:

pipe: wakeup wr_wait after setting max_usage

Commit c73be61cede5 ("pipe: Add general notification queue support") a

regression was introduced that would lock up resized pipes under certain

conditions. See the reproducer in [1].

The commit resizing the pipe ring size was moved to a different

function, doing that moved the wakeup for pipe->wr_wait before actually

raising pipe->max_usage. If a pipe was full before the resize occured it

would result in the wakeup never actually triggering pipe_write.

Set @max_usage and @nr_accounted before waking writers if this isn't a

watch queue.

[Christian Brauner <[email protected]>: rewrite to account for watch queues]