pidfs: allow retrieval of namespace file descriptors

For users that hold a reference to a pidfd procfs might not even be
available nor is it desirable to parse through procfs just for the sake
of getting namespace file descriptors for a process.

Make it possible to directly retrieve namespace file descriptors from a
pidfd. Pidfds already can be used with setns() to change a set of
namespaces atomically.

Link: https://lore.kernel.org/r/20240627-work-pidfs-v1-4-7e9ab6cc3bb1@kernel.org


Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>