f2fs: remove debufs dir if debugfs_create_file() failed

When debugfs_create_file() failed in f2fs_create_root_stats(),
debugfs_root should be remove.

Signed-off-by: Younger Liu <liuyiyang@hisense.com>
Cc: Younger Liu <younger.liucn@gmail.com>
Cc: Jaegeuk Kim <jaegeuk.kim@samsung.com>
Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>