dm vdo: add geometry block structure

dm vdo: add geometry block structure

Introduce a vdo_geometry_block structure, containing a vio and buffer,
mirroring the existing vdo_super_block structure. Both are now
initialized at VDO startup and freed at shutdown, establishing the
infrastructure needed to read and write the geometry block using the
same mechanisms as the super block.

Refactor read_geometry_block() to use the new structure.

Signed-off-by: Bruce Johnston <bjohnsto@redhat.com>
Reviewed-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>