Revert "Revert "dm bufio: subtract the number of initial sectors..."

Revert submission 1672946

Reason for revert: getting back to mainline with a proper fix
Reverted Changes:
I96ee9a0a8:Revert "dm bufio: subtract the number of initial s...
I23d4da47b:Revert "dm verity: fix FEC for RS roots unaligned ...

Change-Id: Ie37a318c2a2fd61969f1102452b47b4e376071d9