linuxkpi: Change `strscpy()` and `strscpy_pad()` to make their `len` argument optional

[dumbbell]

Summary

The previous implementation always took the len but now, it is optional and defaults to the size of dst.

The DRM drivers started to use strscpy() without the len in Linux 6.13.

This is part of the update of DRM drivers to Linux 6.13.

Sponsored by: The FreeBSD Foundation