net: ip_tunnel: use strscpy to replace strlcpy

The strlcpy should not be used because it doesn't limit the source
length. Preferred is strscpy.

Signed-off-by: XueBing Chen <chenxuebing@jari.cn>
Link: https://lore.kernel.org/r/2a08f6c1.e30.181ed8b49ad.Coremail.chenxuebing@jari.cn


Signed-off-by: Jakub Kicinski <kuba@kernel.org>