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 <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #15721