aio: fix a user triggered use after free

[Christoph Hellwig]

Hi Linus,

the patch below fixes a use after free triggered if one threads closes
a file descriptor after another just started an aio operation.

The patch is originally from Jan with some updates from me - I tried
to get in through Al but I haven't managed to get him to respond yet,
but I'd really like to get into the tree, -stable and the distros ASAP.