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.