s3-aio-fork: Fix a segfault in vfs_aio_fork

s3-aio-fork: Fix a segfault in vfs_aio_fork

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.

This patch fixes it in a pretty bad way by introducing flag variables and more
state. But the mid-term plan is to replace the posix aio calls from the vfs and
do pread_send/recv and pwrite_send/recv at the vfs layer, so this will
significantly change anyway.

Thanks to Kirill Malkin <kirill.malkin@starboardstorage.com> for reporting this
crash!

The last 2 patches address bug #8836 (aio_fork segfaults on "smbcontrol
close-share").