zstream decompress: fix decompress size and output

This was incorrectly using the compressed length for the size of the
decompress buffer, and quietly doing nothing if the decompressor refused
to decompress the block because there wasn't enough space.

After that, it wasn't correctly rewriting the record to indicate
"not compressed".

So that's fixed now. Sigh.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>