s390/monwriter: fix gcc 8 stringop-truncation warning

The following gcc warning is issued for strncpy which is used to
deliberately avoid string NUL-termination. Reuse memcpy to avoid the
warning.

    inlined from 'monwrite_diag' at drivers/s390/char/monwriter.c:64:2:
./include/linux/string.h:246:9: warning: '__builtin_strncpy' output
truncated before terminating nul copying 7 bytes from a string of the
same length [-Wstringop-truncation]

Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>