CVE-2024-35925 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.