btrfs: fix uninitialized variable warnings in __set_extent_bit and convert_extent_bit

We will pass in the parent and p pointer into our tree_search function
to avoid doing a second search when inserting a new extent state into
the tree.  However because this is conditional upon passing in these
pointers the compiler seems to think these values can be uninitialized
if we're using -Wmaybe-uninitialized.  Fix this by initializing these
values.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-io-tree.c

@@ -972,8 +972,8 @@ 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;
@@ -1218,8 +1218,8 @@ int convert_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;