tun: replace strcpy with strscpy for ifr_name

tun: replace strcpy with strscpy for ifr_name

Replace the strcpy() calls that copy the device name into ifr->ifr_name
with strscpy() to avoid potential overflows and guarantee NULL termination.

Destination is ifr->ifr_name (size IFNAMSIZ).

Tested in QEMU (BusyBox rootfs):
 - Created TUN devices via TUNSETIFF helper
 - Set addresses and brought links up
 - Verified long interface names are safely truncated (IFNAMSIZ-1)

Signed-off-by: Miguel García <miguelgarciaroman8@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250812082244.60240-1-miguelgarciaroman8@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>