block: prevent division by zero in blk_rq_stat_sum()

In the Linux kernel, the following vulnerability has been resolved: block: prevent division by zero in blk_rq_stat_sum() The expression dst->nr_samples + src->nr_samples may have zero value on overflow. It is necessary to add a check to avoid division by zero. Found by Linux Verification Center (linuxtesting.org) with Svace.