fs/pipe: use spinlock in pipe_read() only if there is a watch_queue

If there is no watch_queue, holding the pipe mutex is enough to
prevent concurrent writes, and we can avoid the spinlock.

O_NOTIFICATION_QUEUE is an exotic and rarely used feature, and of all
the pipes that exist at any given time, only very few actually have a
watch_queue, therefore it appears worthwile to optimize the common
case.

This patch does not optimize pipe_resize_ring() where the spinlocks
could be avoided as well; that does not seem like a worthwile
optimization because this function is not called often.

Related commits:

- commit 8df44129 ("pipe: Check for ring full inside of the
  spinlock in pipe_write()")
- commit b667b867 ("pipe: Advance tail pointer inside of wait
  spinlock in pipe_read()")
- commit 189b0ddc ("pipe: Fix missing lock in pipe_resize_ring()")

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Message-Id: <20230921075755.1378787-4-max.kellermann@ionos.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>