io_uring/rw: split io_read() into a helper

Commit a08d195b upstream.

Add __io_read() which does the grunt of the work, leaving the completion
side to the new io_read(). No functional changes in this patch.

Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
(cherry picked from commit a08d195b)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>