[S390] cio: fix wrong buffer access in cio_ignore_write

Writing only spaces to /proc/cio_ignore will cause a buffer overflow
since the size_t value i will not become negative and so buf[-1UL] is
accessed. Change the value of i to ssize_t.
Signed-off-by: NSebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/cio/blacklist.c

@@ -336,8 +336,7 @@ cio_ignore_write(struct file *file, const char __user *user_buf,
 		 size_t user_len, loff_t *offset)
 {
 	char *buf;
-	size_t i;
-	ssize_t rc, ret;
+	ssize_t rc, ret, i;
 
 	if (*offset)
 		return -EINVAL;