[S390] call home: fix string length handling

After copying uts->nodename to the static nodename array the static
version isn't necessarily zero termininated, since the size of the
array is one byte too short.
Afterwards doing strncat(data, nodename, strlen(nodename)); may copy
an arbitrary large amount of bytes.
Fix this by getting rid of the static array and using strncat with
proper length limit.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>