pipe_read: don't wake up the writer if the pipe is still full

pipe_read: don't wake up the writer if the pipe is still full

wake_up(pipe->wr_wait) makes no sense if pipe_full() is still true after
the reading, the writer sleeping in wait_event(wr_wait, pipe_writable())
will check the pipe_writable() == !pipe_full() condition and sleep again.

Only wake the writer if we actually released a pipe buf, and the pipe was
full before we did so.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/all/20241229135737.GA3293@redhat.com/
Link: https://lore.kernel.org/r/20250102140715.GA7091@redhat.com
Reported-by: WangYuli <wangyuli@uniontech.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>