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.

Fixes: c6385c0 ("netdevsim: Allow reporting activity on nexthop buckets")
Signed-off-by: Zichen Xie <zichenxie0106@gmail.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20241022171907.8606-1-zichenxie0106@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Gax-c

drivers/net/netdevsim/fib.c

@@ -1377,10 +1377,12 @@ static ssize_t nsim_nexthop_bucket_activity_write(struct file *file,
 
 	if (pos != 0)
 		return -EINVAL;
-	if (size > sizeof(buf))
+	if (size > sizeof(buf) - 1)
 		return -EINVAL;
 	if (copy_from_user(buf, user_buf, size))
 		return -EFAULT;
+	buf[size] = 0;
+
 	if (sscanf(buf, "%u %hu", &nhid, &bucket_index) != 2)
 		return -EINVAL;