Use mutex_owned() to Validate and Document Locking Requirements

The syntax for mutex_owned(9F) is as follows:

int mutex_owned(kmutex_t *mp);

A significant portion of driver development involves properly handling multiple threads. Comments should always be used when a mutex is acquired. Comments can be even more useful when an apparently necessary mutex is not acquired. To determine whether a mutex is held by a thread, use mutex_owned() within ASSERT(9F):

void helper(void)
{
    /* this routine should always be called with xsp's mutex held */
    ASSERT(mutex_owned(&xsp->mu));
    /* ... */
}