lightnvm: store rrpc->soffset in device sector size

Since we mainly use soffset in device sector size, we therefore store
this value in rrpc->soffset, instead of the offset in 512byte sector
size. This eliminates the "(ilog2(dev->sec_size) - 9)" calculation on
each I/O.

Signed-off-by: Wenwei Tao <ww.tao0320@gmail.com>
Updated patch description.
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>