Extract v1_read_bitmap() from v1_verify()

Also create read_bitmap_byte() to be reused with format 0002.

Signed-off-by: Patryck Rouleau <pfrouleau@gmail.com>
Extract generate_usedblocks_map() from v1_verify()

Signed-off-by: Patryck Rouleau <pfrouleau@gmail.com>