f2fs: fix to put root inode in error path of fill_super

We should put root inode correctly in error path of fill_super, otherwise we
may encounter a leak case of inode resource.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Reviewed-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>