UPSTREAM: usb: gadget: f_fs: Fix use-after-free

UPSTREAM: usb: gadget: f_fs: Fix use-after-free

(cherry picked from commit 38740a5b87d53ceb89eb2c970150f6e94e00373a)

When using asynchronous read or write operations on the USB endpoints the
issuer of the IO request is notified by calling the ki_complete() callback
of the submitted kiocb when the URB has been completed.

Calling this ki_complete() callback will free kiocb. Make sure that the
structure is no longer accessed beyond that point, otherwise undefined
behaviour might occur.

Fixes: 2e4c7553cd6f ("usb: gadget: f_fs: add aio support")
Cc: <stable@vger.kernel.org> # v3.15+
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Change-Id: I3c7b643f6440c4fb6160a57c1058523030b46a6c
Bug: 30950866