fs/pipe: move check to pipe_has_watch_queue()

This declutters the code by reducing the number of #ifdefs and makes
the watch_queue checks simpler.  This has no runtime effect; the
machine code is identical.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Message-Id: <20230921075755.1378787-2-max.kellermann@ionos.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>