FreeBSD: Fix regression from kmem_scnprintf() in libzfs

kmem_scnprintf() is only available in libzpool. Recent buildbot issues
with showing FreeBSD results kept us from seeing this before
97143b9d314d54409244f3995576d8cc8c1ebf0a was merged.

The code has been changed to sanitize the output from kmem_scnprintf().

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14111