9pfs: xattrcreate requires non-opened fids

The xattrcreate operation only makes sense on a freshly cloned fid
actually, since any open state would be leaked because of the fid_type
change. This is indeed what the linux kernel client does:

	fid = clone_fid(fid);
	[...]
	retval = p9_client_xattrcreate(fid, name, value_len, flags);

This patch also reverts commit ff55e94d since we are sure that a fid
with type P9_FID_NONE doesn't have a previously allocated xattr.
Signed-off-by: NGreg Kurz <groug@kaod.org>

hw/9pfs/9p.c

@@ -3272,6 +3272,11 @@ static void coroutine_fn v9fs_xattrcreate(void *opaque)
         err = -EINVAL;
         goto out_nofid;
     }
+    if (file_fidp->fid_type != P9_FID_NONE) {
+        err = -EINVAL;
+        goto out_put_fid;
+    }
+
     /* Make the file fid point to xattr */
     xattr_fidp = file_fidp;
     xattr_fidp->fid_type = P9_FID_XATTR;
@@ -3281,9 +3286,9 @@ static void coroutine_fn v9fs_xattrcreate(void *opaque)
     xattr_fidp->fs.xattr.flags = flags;
     v9fs_string_init(&xattr_fidp->fs.xattr.name);
     v9fs_string_copy(&xattr_fidp->fs.xattr.name, &name);
-    g_free(xattr_fidp->fs.xattr.value);
     xattr_fidp->fs.xattr.value = g_malloc0(size);
     err = offset;
+out_put_fid:
     put_fid(pdu, file_fidp);
 out_nofid:
     pdu_complete(pdu, err);