btrfs: fix uninitialized variable in find_first_clear_extent_bit

This was caught when syncing extent-io-tree.c into btrfs-progs.  This
however isn't really a problem, the only way next would be uninitialized
is if we found the range we were looking for, and in this case we don't
care about next.  However it's a compile error, so fix it up.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-io-tree.c

@@ -1425,7 +1425,7 @@ void find_first_clear_extent_bit(struct extent_io_tree *tree, u64 start,
 				 u64 *start_ret, u64 *end_ret, u32 bits)
 {
 	struct extent_state *state;
-	struct extent_state *prev = NULL, *next;
+	struct extent_state *prev = NULL, *next = NULL;
 
 	spin_lock(&tree->lock);