ovl: allow idmapping overlay mounts

Now that every overlay inode operation honors the overlay mount idmap,
allow the merged overlay mount itself to be idmapped by setting
FS_ALLOW_IDMAP on ovl_fs_type.  mount_setattr(MOUNT_ATTR_IDMAP) can
then apply an idmapping to an overlay mount, exposing the merged tree
under a different ownership view.

The composition is clean because overlayfs already normalizes every
underlying id through the relevant layer idmap when it copies
attributes into the overlay inode (ovl_copyattr()); the overlay inode's
i_{u,g}id are thus "overlay-final" ids.  The overlay mount idmap
composes on top of that pivot: it is applied to (getattr) and removed
from (setattr, create) those ids at the overlay-inode boundary -
permission, getattr, setattr, ACL owner checks and inode_init_owner() -
while the underlying layers keep being accessed with the mounter's
credentials through their own, possibly idmapped, mounts.  The mount
idmap therefore only changes how the caller sees the overlay inode and
never widens the mounter's access to the layers.

This is deliberately the final code patch of the series: only once
every operation honors the mount idmap is it safe to make overlay
mounts idmappable.

Link: https://patch.msgid.link/20260615-work-idmapped-overlayfs-v1-6-7381632aa402@kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

fs/overlayfs/super.c

@@ -1573,7 +1573,7 @@ struct file_system_type ovl_fs_type = {
 	.name			= "overlay",
 	.init_fs_context	= ovl_init_fs_context,
 	.parameters		= ovl_parameter_spec,
-	.fs_flags		= FS_USERNS_MOUNT,
+	.fs_flags		= FS_USERNS_MOUNT | FS_ALLOW_IDMAP,
 	.kill_sb		= kill_anon_super,
 };
 MODULE_ALIAS_FS("overlay");