This patch adds AIO support to gadgetfs, letting user mode programs use
"libaio" to manage USB I/O concurrency with the same API as they may
already be using for disk files.  In particular, it's practical to
stream isochronous data to/from userspace using this API, by keeping
an endpoint's I/O queue from emptying.

Each AIO "iocb" in userspace corresponds directly to one "usb_request"
(and one kiocb) in the kernel.  Their lifecycles, including cancelation,
overlap completely.  That's much of why the patch is so small (surprised
the heck out of me!); that, and using copy_{to,from}_user() rather than
trying fancy dma mapping tricks to attain zerocopy nirvana.

The kernel AIO module forgot to export kick_iocb(), so this adds the
missing declaration ... needed when using gadgetfs as a module.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>