sysctl: don't overflow the user-supplied buffer with '\0'

If the string was too long to fit in the user-supplied buffer,
the sysctl layer would zero-terminate it by writing past the
end of the buffer. Don't do that.

Noticed by Yi Yang <[e-mail]>

Signed-off-by: Linus Torvalds <[e-mail]>

kernel/sysctl.c

···
 		if (get_user(len, oldlenp))
 			return -EFAULT;
 		if (len) {
-			l = strlen(table->data);
+			l = strlen(table->data)+1;
 			if (len > l) len = l;
 			if (len >= table->maxlen)
 				len = table->maxlen;
 			if(copy_to_user(oldval, table->data, len))
-				return -EFAULT;
-			if(put_user(0, ((char __user *) oldval) + len))
 				return -EFAULT;
 			if(put_user(len, oldlenp))
 				return -EFAULT;