dm-verity-fec: replace io_size with block_size

dm-verity-fec: replace io_size with block_size

dm-verity's FEC implementation assumes that data_block_size ==
hash_block_size, and it accesses the FEC device in units of the same
size.  Many places in the code want that size and compute it on-demand
as '1 << v->data_dev_block_bits'.  However, it's actually already
available in v->fec->io_size.  Rename that field to block_size,
initialize it a bit earlier, and use it in the appropriate places.

Note that while these sizes could in principle be different, that case
is not supported.  So there's no need to complicate the code for it.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>