io_uring: inline io_uring_attempt_task_drop()

[ Upstream commit 4f793dc4 ]

A simple preparation change inlining io_uring_attempt_task_drop() into
io_uring_flush().

Cc: stable@vger.kernel.org # 5.5+
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>