Fix zstream_t incorrect type

The DMU zfetch code organizes streams with lists not avl trees.  A
avl_node_t was mistakenly used for a list_node_t in the zstream_t
type.  This is incorrect (but harmless) and when unnoticed because:

1) The list functions explicitly cast the value preventing a warning,
2) sizeof(avl_node_t) >= sizeof(list_node_t) so no overrun occurs, and
3) The calculated offset is the same regardless of the type.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1946

include/sys/dmu_zfetch.h

@@ -50,7 +50,7 @@ typedef struct zstream {
 	uint64_t	zst_cap;	/* prefetch limit (cap), in blocks */
 	kmutex_t	zst_lock;	/* protects stream */
 	clock_t		zst_last;	/* lbolt of last prefetch */
-	avl_node_t	zst_node;	/* embed avl node here */
+	list_node_t	zst_node;	/* next zstream here */
 } zstream_t;
 
 typedef struct zfetch {