btrfs: fix double free in create_space_info_sub_group() error path

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

btrfs: fix double free in create_space_info_sub_group() error path

When kobject_init_and_add() fails, the call chain is:

create_space_info_sub_group()

-> btrfs_sysfs_add_space_info_type()

-> kobject_init_and_add()

-> failure

-> kobject_put(&sub_group->kobj)

-> space_info_release()

-> kfree(sub_group)

Then control returns to create_space_info_sub_group(), where:

btrfs_sysfs_add_space_info_type() returns error

-> kfree(sub_group)

Thus, sub_group is freed twice.

Keep parent->sub_group[index] = NULL for the failure path, but after

btrfs_sysfs_add_space_info_type() has called kobject_put(), let the

kobject release callback handle the cleanup.