s390: Remove optional third argument of strscpy() if possible

The third argument of strscpy() is optional and can be left away iff
the destination is an array and the maximum size of the copy is the
size of destination.
Remove the third argument for those cases where this is possible.

Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>