librbd: make group and group snapshot IDs more random

librbd: make group and group snapshot IDs more random

Image IDs suffered from the same issue -- it was addressed in commit
be8373688c1b ("librbd: block_name_prefix is not created randomly").
The code for generating group IDs is duplicated in api/Group.cc and
got missed.

Instead of cut-and-pasting the fix, just call generate_image_id()
directly and rename variables for more explicitness.

Before:

  $ rados -p rbd ls | grep rbd_group_header
  rbd_group_header.10256b8b4567
  rbd_group_header.10216b8b4567
  rbd_group_header.10236b8b4567
  rbd_group_header.101f6b8b4567

After:

  $ rados -p rbd ls | grep rbd_group_header
  rbd_group_header.10255f555a5
  rbd_group_header.1023f347eafb
  rbd_group_header.101f24c75111
  rbd_group_header.1021dda4e122

Fixes: https://tracker.ceph.com/issues/65573
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>