writer: Ensure re-setting xattrs are re-validated

We did a lot of work to limit the size of xattrs, but
honggfuzz quickly figured out it was possible to bypass
this by specifying an xattr multiple times; the second
copy had special "replace existing value" code that
didn't check limits.

Fix this by always calling the "remove xattr" API when
setting (but ignoring the value). This ensures we
are consistently tracking limits.

Signed-off-by: Colin Walters <walters@verbum.org>

libcomposefs/lcfs-writer.c

@@ -1676,27 +1676,14 @@ int lcfs_node_set_xattr_internal(struct lcfs_node_s *node, const char *name,
 		return -1;
 	}
 
-	ssize_t index = find_xattr(node, name);
-
 	if (value_len > UINT16_MAX) {
 		errno = EINVAL;
 		return -1;
 	}
 
-	if (index >= 0) {
-		/* Already set, replace */
-		struct lcfs_xattr_s *xattr = &node->xattrs[index];
-		v = memdup(value, value_len);
-		if (v == NULL) {
-			errno = ENOMEM;
-			return -1;
-		}
-		free(xattr->value);
-		xattr->value = v;
-		xattr->value_len = value_len;
+	// Remove any existing value
+	(void)lcfs_node_unset_xattr(node, name);
 
-		return 0;
-	}
 	// Double the xattr metadata size, subtracting 1 to account for worst case alignment.
 	size_t entry_size = (2 * LCFS_INODE_XATTRMETA_SIZE) - 1 + namelen + value_len;
 	// If this is the first xattr, add size for the header