dm vdo indexer: reorder uds_request to reduce padding

Reorder fields and make uds_request_type and uds_zone_message packed,
to squeeze out some space. Use struct_group so the request reset code
no longer needs to care about field order.

On x86_64 this reduces the struct size from 144 to 120, which saves 48
kB (about 12%) per VDO hash zone.

Signed-off-by: Ken Raeburn <raeburn@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>