dm vdo: save the formatted metadata to disk

dm vdo: save the formatted metadata to disk

Add vdo_save_super_block() and vdo_save_geometry_block() to perform
asynchronous writes of the super block and geometry block respectively.
Add vdo_clear_layout() to zero the UDS index's first block, the block
map partition, and the recovery journal partition.

These operations are driven by new phases in the pre-load state machine
(PRE_LOAD_PHASE_FORMAT_*), ensuring that disk writes happen during
pre-resume rather than during dmsetup create.

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