kobject: Add sanity check for kset->kobj.ktype in kset_register()

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

kobject: Add sanity check for kset->kobj.ktype in kset_register()

When I register a kset in the following way:

static struct kset my_kset;

kobject_set_name(&my_kset.kobj, "my_kset");

ret = kset_register(&my_kset);

A null pointer dereference exception is occurred:

[ 4453.568337] Unable to handle kernel NULL pointer dereference at \

virtual address 0000000000000028

... ...

[ 4453.810361] Call trace:

[ 4453.813062] kobject_get_ownership+0xc/0x34

[ 4453.817493] kobject_add_internal+0x98/0x274

[ 4453.822005] kset_register+0x5c/0xb4

[ 4453.825820] my_kobj_init+0x44/0x1000 [my_kset]

... ...

Because I didn't initialize my_kset.kobj.ktype.

According to the description in Documentation/core-api/kobject.rst:

- A ktype is the type of object that embeds a kobject. Every structure

that embeds a kobject needs a corresponding ktype.

So add sanity check to make sure kset->kobj.ktype is not NULL.