dm vdo: rework processing of loaded refcount byte arrays

Clear provisional refcount values and count free/allocated blocks in
one integrated loop. Process 8 aligned bytes at a time instead of
every byte individually.

On an Intel i7-11850H this reduces the CPU time needed to process a
loaded refcount block by a factor of about 5-6. On a large system the
refcount loading may be the largest factor in device startup time.

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