Free property names with spa_strfree() rather than strfree()

Since they're allocated with spa_strdup(), they should be freed with
spa_strfree() so the proper length buffer is freed.

Reviewed-by: Richard Yao <ryao@gentoo.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #5082
Closes #5086