libfs: prepare to allow for non-immutable pidfd inodes

Allow for S_IMMUTABLE to be stripped so that we can support xattrs.

Link: https://lore.kernel.org/20250618-work-pidfs-persistent-v2-10-98f3456fd552@kernel.org
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

fs/libfs.c

@@ -2162,7 +2162,6 @@ static struct dentry *prepare_anon_dentry(struct dentry **stashed,
 
 	/* Notice when this is changed. */
 	WARN_ON_ONCE(!S_ISREG(inode->i_mode));
-	WARN_ON_ONCE(!IS_IMMUTABLE(inode));
 
 	dentry = d_alloc_anon(sb);
 	if (!dentry) {