fhandle, pidfs: support open_by_handle_at() purely based on file handle

Various filesystems such as pidfs (and likely drm in the future) have a
use-case to support opening files purely based on the handle without
having to require a file descriptor to another object. That's especially
the case for filesystems that don't do any lookup whatsoever and there's
zero relationship between the objects. Such filesystems are also
singletons that stay around for the lifetime of the system meaning that
they can be uniquely identified and accessed purely based on the file
handle type. Enable that so that userspace doesn't have to allocate an
object needlessly especially if they can't do that for whatever reason.

Link: https://lore.kernel.org/20250624-work-pidfs-fhandle-v2-10-d02a04858fe3@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/fhandle.c

@@ -188,6 +188,11 @@ static int get_path_anchor(int fd, struct path *root)
 		return 0;
 	}
 
+	if (fd == FD_PIDFS_ROOT) {
+		pidfs_get_root(root);
+		return 0;
+	}
+
 	return -EBADF;
 }