ksmbd: use list_first_entry_or_null for opinfo_get_list()

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

ksmbd: use list_first_entry_or_null for opinfo_get_list()

The list_first_entry() macro never returns NULL. If the list is

empty then it returns an invalid pointer. Use list_first_entry_or_null()

to check if the list is empty.