rcu/trace: use strscpy() to instead of strncpy()

This commit saves a line of code by switching from strncpy() to strscpy()
by permitting the later NUL assignment to be removed.  While in the area,
save another line by taking advantage of 100 characters.

Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Xu Panda <xu.panda@zte.com.cn>
Signed-off-by: Yang Yang <yang.yang29@zte.com.cn>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>