CVE-2021-47470 mm, slub: fix potential use-after-free in slab_debugfs_fops

In the Linux kernel, the following vulnerability has been resolved:

mm, slub: fix potential use-after-free in slab_debugfs_fops

When sysfs_slab_add failed, we shouldn’t call debugfs_slab_add() for s
because s will be freed soon. And slab_debugfs_fops will use s later
leading to a use-after-free.