Coverity 9649, 9650, 9651: Uninit

This check was originally added to detect double initializations
of mutex types (which it did find).  Unfortunately, Coverity is
right that there is a very small chance we could trigger the
assertion by accident because an uninitialized stack variable
happens to contain the mutex magic.  This is particularly unlikely
since we do poison the mutexs when destroyed but still possible.
Therefore I'm simply removing the assertion.

module/spl/spl-mutex.c

@@ -67,7 +67,6 @@ __spl_mutex_init(kmutex_t *mp, char *name, int type, void *ibc)
 	ASSERT(mp);
 	ASSERT(name);
 	ASSERT(ibc == NULL);
-	ASSERT(mp->km_magic != KM_MAGIC); /* Never double init */
 
 	mp->km_name = NULL;
 	mp->km_name_size = strlen(name) + 1;