Stop wasting time on malloc in snprintf_zstd_header

Profiling zdb -vvvvv on datasets with a lot of zstd blocks, we find
ourselves spending quite a lot of time on malloc/free, because we
allocate a 16M abd each call, and never free it, so we're leaking
16M per call as well.

This seems sub-optimal. So let's just keep the buffer around and
reuse it.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Rob Norris <[email protected]>
Signed-off-by: Rich Ercolani <[email protected]>
Closes #15721 

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