btrfs: fix an uninitialized variable warning in btrfs_log_inode

This fixes the following warning reported by gcc 10.2.1 under x86_64:

../fs/btrfs/tree-log.c: In function ‘btrfs_log_inode’:
../fs/btrfs/tree-log.c:6211:9: error: ‘last_range_start’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
 6211 |   ret = insert_dir_log_key(trans, log, path, key.objectid,
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 6212 |       first_dir_index, last_dir_index);
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../fs/btrfs/tree-log.c:6161:6: note: ‘last_range_start’ was declared here
 6161 |  u64 last_range_start;
      |      ^~~~~~~~~~~~~~~~

This might be a false positive fixed in later compiler versions but we
want to have it fixed.
Reported-by: Nk2ci <kernel-bot@kylinos.cn>
Reviewed-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NShida Zhang <zhangshida@kylinos.cn>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/tree-log.c

@@ -6158,7 +6158,7 @@ static int log_delayed_deletions_incremental(struct btrfs_trans_handle *trans,
 {
 	struct btrfs_root *log = inode->root->log_root;
 	const struct btrfs_delayed_item *curr;
-	u64 last_range_start;
+	u64 last_range_start = 0;
 	u64 last_range_end = 0;
 	struct btrfs_key key;