treewide: correctly apply IFNAMSIZ limit

treewide: correctly apply IFNAMSIZ limit

The `IFNAMSIZ` macro defines the required buffer size to hold a Linux
interface name including the terminating zero byte while netifd currently
uses an `IFNAMSIZ + 1` limit for interface name buffers.

This causes netifd to use overlong names (16 instead of 15 bytes) in
netlink communication with the kernel, leading to netlink failure replies
due to policy violations.

Fix this issue by applying the correct length, that is `IFNAMSIZ` directly,
to the corresponding buffers.

Ref: https://github.com/openwrt/openwrt/issues/11259
Signed-off-by: Jo-Philipp Wich <jo@mein.io>