poll_wait: add mb() to fix theoretical race between waitqueue_active() and .poll()

As the comment above waitqueue_active() explains, it can only be used
if both waker and waiter have mb()'s that pair with each other. However
__pollwait() is broken in this respect.

This is not pipe-specific, but let's look at pipe_poll() for example:

poll_wait(...); // -> __pollwait() -> add_wait_queue()

LOAD(pipe->head);
LOAD(pipe->head);

In theory these LOAD()'s can leak into the critical section inside
add_wait_queue() and can happen before list_add(entry, wq_head), in this
case pipe_poll() can race with wakeup_pipe_readers/writers which do

smp_mb();
if (waitqueue_active(wq_head))
wake_up_interruptible(wq_head);

There are more __pollwait()-like functions (grep init_poll_funcptr), and
it seems that at least ep_ptable_queue_proc() has the same problem, so the
patch adds smp_mb() into poll_wait().

Link: https://lore.kernel.org/all/20250102163320.GA17691@redhat.com/
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20250107162717.GA18922@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>