As commit 8f7acdae2cd4 ("staging: erofs: kill all failure handling in
fill_super()"), move the initialization of packed inode after root
inode is assigned, so that the iput() in .put_super() is adequate as
the failure handling.
Otherwise, iput() is also needed in .kill_sb(), in case of the mounting
fails halfway.
Signed-off-by: Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx>