staging: erofs: kill all failure handling in fill_super()

.kill_sb() will do that instead in order to remove duplicated code.

Note that the initialzation of managed_cache is now moved
after s_root is assigned since it's more preferred to iput()
in .put_super() and all inodes should be evicted before
the end of generic_shutdown_super(sb).

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190731155752.210602-12-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>