nstree: allow lookup solely based on inode

nstree: allow lookup solely based on inode

The namespace file handle struct nsfs_file_handle is uapi and userspace
is expressly allowed to generate file handles without going through
name_to_handle_at().

Allow userspace to generate a file handle where both the inode number
and the namespace type are zero and just pass in the unique namespace
id. The kernel uses the unified namespace tree to find the namespace and
open the file handle.

When the kernel creates a file handle via name_to_handle_at() it will
always fill in the type and the inode number allowing userspace to
retrieve core information.

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-14-2e6f823ebdc0@kernel.org
Tested-by: syzbot@syzkaller.appspotmail.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>