s390/ipl: Do not accept z/VM CP diag X'008' cmds longer than max length

The old implementation of vmcmd sysfs string attributes truncated passed
z/VM CP diagnose X'008' commands which were longer than the max allowed
number of characters but the reported number of written characters was
still equal to the entire length of a given string. This can result in
silent failures of some s390-tools (e.g. dumpconf) which can be very hard
to detect. Therefore, this commit makes a write attempt to a vmcmd sysfs
attribute
* fail with E2BIG error if a given string is longer than the maximum
  allowed one
* never destroy the old data in the vmcmd sysfs attribute if the new data
  doesn't fit into it entirely
* return the actual number of written characters if it succeeds
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/kernel/ipl.c

@@ -266,7 +266,11 @@ static ssize_t sys_##_prefix##_##_name##_store(struct kobject *kobj,	\
 		struct kobj_attribute *attr,				\
 		const char *buf, size_t len)				\
 {									\
-	strscpy(_value, buf, sizeof(_value));				\
+	if (len >= sizeof(_value))					\
+		return -E2BIG;						\
+	len = strscpy(_value, buf, sizeof(_value));			\
+	if (len < 0)							\
+		return len;						\
 	strim(_value);							\
 	return len;							\
 }									\