Fix the same bug reported by Kirill Malkin <kirill.malkin@starboardstorage.com> and

Fix the same bug reported by Kirill Malkin <kirill.malkin@starboardstorage.com>  and
fixed by Volker for vfs_aio_fork as ref 0aacdbfada46329e0ad9dacfa90041a1c7dbf3e8.

From that change:

    aio_suspend does not signal the main process with a signal, it just waits. The
    aio_fork module does not use the signal at all, it directly calls back into the
    main smbd by calling smbd_aio_complete_aio_ex. This is an abstraction
    violation, but the alternative would have been to use signals where they are
    not needed. However, in wait_for_aio_completion this bites us: With aio_fork we
    call handle_aio_completed twice on the same aio_ex struct: Once from the call
    to handle_aio_completion within the aio_fork module and once from the code in
    wait_for_aio_completion.

Fix this differently here by not calling directly back into smbd,
but using a new function aio_linux_setup_returns() to setup the
return values that wait_for_aio_completion() in the main smbd
will pick up by calling handle_aio_completd().