Fix uninitialized variable in avl_add()

Fix uninitialized variable in avl_add()

Silence the following warning when compiling with gcc 5.4.0.
Specifically gcc (Ubuntu 5.4.0-6ubuntu1~16.04.1) 5.4.0 20160609.

module/avl/avl.c: In function ‘avl_add’:
module/avl/avl.c:647:2: warning: ‘where’ may be used uninitialized
    in this function [-Wmaybe-uninitialized]
  avl_insert(tree, new_node, where);

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