fix bugs related to introduction of flag --blocksize_kb

[emillynge]

Dear maintainers

Please accept this PR which fixes a couple of issues that has arisen due to the introduction of --blocksize_kb option.

In general these issue stem from the previous assumption that a block is 1MB.

• The stats MBps and MB are wrong when bz!=1MB
• Files are not rotated at the right time. Specifically, bz>1MB can lead to a PKT file larger than 4Gb which causes a fatal error.

To fix the issue, i've introduced a new variable blocks_per_file which is determined by the values flag_filesize_mband flag_blocksize_kb.

I'm not entirely sure whether the pattern of calculating blocks_per_file in the switch block is the right way to go.

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