no limitation for "%s" while calling fscanf()

[QiAnXinCodeSafe]

https://github.com/facebook/fboss/blob/8865126044e7a6647c2ba7b0aa0c2d974edeb1eb/fboss/util/nic_util.cpp#L494

[iris577]

@bdd

[slogan621]

Not only that, but sprintf on the line before would be more secure as snprintf, and strcat on the line after would be more secure as strncat.