netdevsim: Add trailing zero to terminate the string in nsim_nexthop_bucket_activity_write()

In the Linux kernel, the following vulnerability has been resolved: netdevsim: Add trailing zero to terminate the string in nsim_nexthop_bucket_activity_write() This was found by a static analyzer. We should not forget the trailing zero after copy_from_user() if we will further do some string operations, sscanf() in this case. Adding a trailing zero will ensure that the function performs properly.