listmount: allow listing in reverse order

util-linux is about to implement listmount() and statmount() support.
Karel requested the ability to scan the mount table in backwards order
because that's what libmount currently does in order to get the latest
mount first. We currently don't support this in listmount(). Add a new
LISTMOUNT_REVERSE flag to allow listing mounts in reverse order. For
example, listing all child mounts of /sys without LISTMOUNT_REVERSE
gives:

    /sys/kernel/security @ mnt_id: 4294968369
    /sys/fs/cgroup @ mnt_id: 4294968370
    /sys/firmware/efi/efivars @ mnt_id: 4294968371
    /sys/fs/bpf @ mnt_id: 4294968372
    /sys/kernel/tracing @ mnt_id: 4294968373
    /sys/kernel/debug @ mnt_id: 4294968374
    /sys/fs/fuse/connections @ mnt_id: 4294968375
    /sys/kernel/config @ mnt_id: 4294968376

whereas with LISTMOUNT_REVERSE it gives:

    /sys/kernel/config @ mnt_id: 4294968376
    /sys/fs/fuse/connections @ mnt_id: 4294968375
    /sys/kernel/debug @ mnt_id: 4294968374
    /sys/kernel/tracing @ mnt_id: 4294968373
    /sys/fs/bpf @ mnt_id: 4294968372
    /sys/firmware/efi/efivars @ mnt_id: 4294968371
    /sys/fs/cgroup @ mnt_id: 4294968370
    /sys/kernel/security @ mnt_id: 4294968369

Link: https://lore.kernel.org/r/20240607-vfs-listmount-reverse-v1-4-7877a2bfa5e5@kernel.org


Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>