io_uring/rw: shrink io_iov_compat_buffer_select_prep

Compat performance is not important and simplicity is more appreciated.
Let's not be smart about it and use simpler copy_from_user() instead of
access + __get_user pair.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/b334a3a5040efa424ded58e4d8a6ef2554324266.1740400452.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>