fix: fix incorrect compact task memory estimation

[hzxa21]

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What's changed and what's your intention?

Prior to this PR, we mistakenly use the max sst meta size in the level / current sstable size to calculate the max meta ratio, which can be very large when there is a small file. This PR fixes the issue by using current sstable meta size / current sstable size to calculate the ratio instead.

Example task that can cause the issue:

Level 5:
SST1: meta_size = 100KB, SST size = 100MB
SST2: meta_size = 100B, SST size = 1KB

max_meta_ratio  = 100KB / 1KB = 100

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[Li0k]

LGTM, thanks for quick fix