target/rd: simplify the page/offset computation

Breakout rd_MEMCPY_do_task() usage of do_div() to tmp value during
rd_request->rd_page assignment.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>