…r size

The ceph_erasure_code_benchmark tool and bench.sh scripts use small default buffer sizes, and also contain comments that suggest running the tool with a buffer size that is too small. It can be observed from flame graphs captured while using the tool that a majority of CPU time is spent allocating, freeing and aligning buffers, rather than testing the performance of the erasure code plugin.

This PR updates the default settings and comments to suggest a far larger buffer size and far fewer number of encode/decode iterations. Flame graphs show that the vast majority of CPU time is now spent encoding and decoding, rather than aligning buffers.

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