lib/libzpool/kernel.c: Assert no owners in rw_destroy()

This is intended to cause ztest to fail when rw_destroy() is called on a
rwlock that has owners.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2330