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