batman-adv: Replace usage of strlcpy with strscpy

The strscpy was introduced to fix some API problems around strlcpy. And
checkpatch started to report recently that strlcpy is deprecated and
strscpy is preferred.

The functionality introduced in commit 30035e45 ("string: provide
strscpy()") improves following points compared to strlcpy:

* it doesn't read from memory beyond (src + size)
* provides an easy way to check for destination buffer overflow
* robust against asynchronous source buffer changes

Since batman-adv doesn't depend on any of the previously mentioned behavior
changes, the usage of strlcpy can simply be replaced by strscpy to silence
checkpatch.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>