Fix --enable-debug-kmem-tracking option

Re-order initialization in spl_kmem_init to allow for kmem tracing
to work.  The spl_kmem_init function calls taskq_create prior to
initializing the tracking (calling spl_kmem_init_tracking).  Since
taskq_create uses kmem_alloc, NULL dereferences occur because the
global kmem_list hasn't had its next & prev pointers initialized yet.

This commit moves the calls to spl_kmem_init_tracking earlier in the
spl_kmem_init function in order that the subsequent kmem_alloc calls
(by taskq_create) work properly.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#243

module/spl/spl-kmem.c

@@ -2418,20 +2418,21 @@ spl_kmem_init(void)
 	int rc = 0;
 	SENTRY;
 
-	init_rwsem(&spl_kmem_cache_sem);
-	INIT_LIST_HEAD(&spl_kmem_cache_list);
-	spl_kmem_cache_taskq = taskq_create("spl_kmem_cache",
-	    1, maxclsyspri, 1, 32, TASKQ_PREPOPULATE);
-
-	spl_register_shrinker(&spl_kmem_cache_shrinker);
-
 #ifdef DEBUG_KMEM
 	kmem_alloc_used_set(0);
 	vmem_alloc_used_set(0);
 
 	spl_kmem_init_tracking(&kmem_list, &kmem_lock, KMEM_TABLE_SIZE);
 	spl_kmem_init_tracking(&vmem_list, &vmem_lock, VMEM_TABLE_SIZE);
 #endif
+
+	init_rwsem(&spl_kmem_cache_sem);
+	INIT_LIST_HEAD(&spl_kmem_cache_list);
+	spl_kmem_cache_taskq = taskq_create("spl_kmem_cache",
+	    1, maxclsyspri, 1, 32, TASKQ_PREPOPULATE);
+
+	spl_register_shrinker(&spl_kmem_cache_shrinker);
+
 	SRETURN(rc);
 }