ublk: cleanup ublk_copy_user_pages

Clean up ublk_copy_user_pages() by using iov_iter_get_pages2, and code
gets simplified a lot and becomes much more readable than before.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230519065030.351216-4-ming.lei@redhat.com


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