Btrfs: Fix typo in clear_state_cb

In clear_state_cb, we should check 'tree->ops->clear_bit_hook' instead
of 'tree->ops->set_bit_hook'.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent_io.c

@@ -340,7 +340,7 @@ static void clear_state_cb(struct extent_io_tree *tree,
 			   struct extent_state *state,
 			   unsigned long bits)
 {
-	if (tree->ops && tree->ops->set_bit_hook) {
+	if (tree->ops && tree->ops->clear_bit_hook) {
 		tree->ops->clear_bit_hook(tree->mapping->host, state->start,
 					  state->end, state->state, bits);
 	}