btrfs: avoid error code override in btrfs_get_acl

It's not good to override the error code when failing from
btrfs_getxattr() in btrfs_get_acl() because it hides the real reason of
the failure.

Signed-off-by: Chengguang Xu <[e-mail]>
Reviewed-by: Nikolay Borisov <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/acl.c

···
 	} else if (size == -ENODATA || size == 0) {
 		acl = NULL;
 	} else {
-		acl = ERR_PTR(-EIO);
+		acl = ERR_PTR(size);
 	}
 	kfree(value);