ubifs: remove error message in ubifs_xattr_get

There is a situation that other modules, like overlayfs, try to get
xattr value with a small buffer, if they get -ERANGE, they will try
again with the proper buffer size. No need to report an error.

Signed-off-by: Rock Lee <rli@sierrawireless.com>
Signed-off-by: Richard Weinberger <richard@nod.at>