staging/lustre: Use proper number of bytes in copy_from_user

This patch removes the usage of MAX_STRING_SIZE from
copy_from_user() and just copies enough bytes to cover
count passed in.
Signed-off-by: Jian Yu <jian.yu@intel.com>
Reviewed-on: http://review.whamcloud.com/23462
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-8774Reviewed-by: John L. Hammond <john.hammond@intel.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/obdclass/lprocfs_status.c

@@ -400,10 +400,17 @@ int lprocfs_wr_uint(struct file *file, const char __user *buffer,
 	char dummy[MAX_STRING_SIZE + 1], *end;
 	unsigned long tmp;
 
-	dummy[MAX_STRING_SIZE] = '\0';
-	if (copy_from_user(dummy, buffer, MAX_STRING_SIZE))
+	if (count >= sizeof(dummy))
+		return -EINVAL;
+
+	if (count == 0)
+		return 0;
+
+	if (copy_from_user(dummy, buffer, count))
 		return -EFAULT;
 
+	dummy[count] = '\0';
+
 	tmp = simple_strtoul(dummy, &end, 0);
 	if (dummy == end)
 		return -EINVAL;