GFS2: Use RB_CLEAR_NODE() rather than rb_init_node()

gfs2 calls RB_EMPTY_NODE() to check if nodes are not on an rbtree.
The corresponding initialization function is RB_CLEAR_NODE().
rb_init_node() was never clearly defined and is going away.

Signed-off-by: Michel Lespinasse <walken@google.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>