PCI: endpoint: Avoid creating sub-groups asynchronously

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

PCI: endpoint: Avoid creating sub-groups asynchronously

The asynchronous creation of sub-groups by a delayed work could lead to a

NULL pointer dereference when the driver directory is removed before the

work completes.

The crash can be easily reproduced with the following commands:

# cd /sys/kernel/config/pci_ep/functions/pci_epf_test

# for i in {1..20}; do mkdir test && rmdir test; done

BUG: kernel NULL pointer dereference, address: 0000000000000088

...

Call Trace:

configfs_register_group+0x3d/0x190

pci_epf_cfs_work+0x41/0x110

process_one_work+0x18f/0x350

worker_thread+0x25a/0x3a0

Fix this issue by using configfs_add_default_group() API which does not

have the deadlock problem as configfs_register_group() and does not require

the delayed work handler.

[mani: slightly reworded the description and added stable list]