Change VERIFY to ASSERT in mutex_destroy()

There have been multiple reports of 'zdb' tripping the VERIFY in
mutex_destroy() because pthread_mutex_destroy() returns EBUSY.

Exactly how this can happen still needs to be explained, but this
doesn't strictly need to be fatal for non-debug builds.  Therefore,
this patch converts the VERIFY to an ASSERT until the root cause
is determined and resolved.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2027