[BLOCK] Clear sg entry before filling in blk_rq_map_sg()

The memset() of the sg entry was originally removed, because it could
overwrite a chain pointer. But it's quite OK to memset() it when we know
it's a valid entry, since it can't contain a chain pointer.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>