Btrfs-progs: mkfs: don't create extent for an empty file

Steps to reproduce:
 # mkdir -p /tmp/test
 # touch /tmp/test/file
 # mkfs.btrfs -f /dev/sda13 -r /tmp/test
 # btrfs check /dev/sda13

For an empty file, don't create extent data for it.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>

mkfs.c

@@ -619,6 +619,9 @@ static int add_file_items(struct btrfs_trans_handle *trans,
 	struct extent_buffer *eb = NULL;
 	int fd;
 
+	if (st->st_size == 0)
+		return 0;
+
 	fd = open(path_name, O_RDONLY);
 	if (fd == -1) {
 		fprintf(stderr, "%s open failed\n", path_name);