Correct level handling in zstream recompress.

sscanf returns number of items parsed on success and EOF on failure.

Reviewed-by: Adam Moss <c@yotes.com>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #16198