UPSTREAM: io_uring: do not recalculate ppos unnecessarily

commit b4aec400 upstream.

There is a slight optimisation to be had by calculating the correct pos
pointer inside io_kiocb_update_pos and then using that later.

It seems code size drops by a bit:
000000000000a1b0 0000000000000400 t io_read
000000000000a5b0 0000000000000319 t io_write

vs
000000000000a1b0 00000000000003f6 t io_read
000000000000a5b0 0000000000000310 t io_write

Change-Id: I19d8cdb6ea88d8fc4625e521363d5a8f638dfdcb
Signed-off-by: Dylan Yudaken <dylany@fb.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Bug: 268174392
(cherry picked from commit e90cfb96)
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>