s390/monwriter: fix smatch warning for strcpy()

This patch fixes the following smatch warning:
monwrite_diag() error: strcpy() '"LNXAPPL"' too large for 'id.prod_nr'
(8 vs 7)
Using strcpy() is wrong, because it also copies the terminating null
byte, but in this case the extra copied null byte will be overwritten
right after the strcpy(), so there is no real problem here.
Use strncpy() to fix the warning.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>