ipmi:msghandler: Fix potential memory corruption in ipmi_create_user()

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

ipmi:msghandler: Fix potential memory corruption in ipmi_create_user()

The "intf" list iterator is an invalid pointer if the correct "intf->intf_num" is not found. Calling atomic_dec(&intf->nr_users) on and invalid pointer will lead to memory corruption.

We don't really need to call atomic_dec() if we haven't called atomic_add_return() so update the if (intf->in_shutdown) path as well.