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.