btrfs-progs: fix may be used uninitialized in __set_extent_bit

Compiler is throwing out this false positive warning in the following
function flow. Apparently, %parent and %p are initialized in tree_search_for_insert().

  __set_extent_bit()
  state = tree_search_for_insert(tree, start, &p, &parent);
  insert_state_fast(tree, prealloc, p, parent, bits, changeset);
   rb_link_node(&state->rb_node, parent, node);

Compile warnings:

In file included from ./common/extent-cache.h:23,
                 from kernel-shared/ctree.h:26,
                 from kernel-shared/extent-io-tree.c:4:
kernel-shared/extent-io-tree.c: In function ‘__set_extent_bit’:
./kernel-lib/rbtree.h:80:28: warning: ‘parent’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  node->__rb_parent_color = (unsigned long)parent;
                            ^~~~~~~~~~~~~~~~~~~~~
kernel-shared/extent-io-tree.c:996:18: note: ‘parent’ was declared here
  struct rb_node *parent;
                  ^~~~~~
In file included from ./common/extent-cache.h:23,
                 from kernel-shared/ctree.h:26,
                 from kernel-shared/extent-io-tree.c:4:
./kernel-lib/rbtree.h:83:11: warning: ‘p’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  *rb_link = node;
  ~~~~~~~~~^~~~~~
kernel-shared/extent-io-tree.c:995:19: note: ‘p’ was declared here
  struct rb_node **p;

Fix:

 Initialize to NULL, as in the kernel.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: asj

kernel-shared/extent-io-tree.c

@@ -993,8 +993,8 @@ static int __set_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
 {
 	struct extent_state *state;
 	struct extent_state *prealloc = NULL;
-	struct rb_node **p;
-	struct rb_node *parent;
+	struct rb_node **p = NULL;
+	struct rb_node *parent = NULL;
 	int err = 0;
 	u64 last_start;
 	u64 last_end;