io_uring: always use original task when preparing req identity

If the ring is setup with IORING_SETUP_IOPOLL and we have more than
one task doing submissions on a ring, we can up in a situation where
we assign the context from the current task rather than the request
originator.

Always use req->task rather than assume it's the same as current.

No upstream patch exists for this issue, as only older kernels with
the non-native workers have this problem.

Reported-by: Kyle Zeng <zengyhkyle@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>