dm-verity-fec: rename rounds to region_blocks

dm-verity-fec: rename rounds to region_blocks

It's hard to reconcile the value stored in dm_verity_fec::rounds with
its name and documentation.  Most likely "rounds" is being used as an
alias for what is more commonly called the interleaving degree or
"number of ways".  But the interleaving is done at the byte level,
whereas the units of "rounds" are blocks.  So it's not really that.

In practice, the reason the code needs this value is that it expresses
the number of blocks in each "region" of the message data, where each
region contains the bytes from a particular index in the RS codewords.

Rename it to region_blocks to make the code a bit more understandable.

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