vfs_aio_fork: Fix a crash in aio_fork

vfs_aio_fork: Fix a crash in aio_fork

Since the introduction of the vfs_aio_fork:erratic_testing_mode this
crashed reliably, as we had two different structs behind
SMB_VFS_HANDLE_SET_DATA. I had always believed that due to the fact that
we have specific aio_fork tests in our autobuild, this would have been
tested. But it was not, because the share definition missed the the "aio
read/write size = 1" to actually use the async code in vfs_aio_fork.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>