dm-verity-fec: simplify computation of ileaved

fec_read_bufs() just iterates over a sequence of message blocks with
step size region_blocks. At each step, 'ileaved' is just the offset (in
bytes) to one of these blocks. Compute it in the straightforward way,
eliminating fec_interleave().

In more detail, previously the code computed
'ileaved = (n / k) + (n % k) * (region_blocks * block_size)'
where n = rsb * k + i and 0 <= i < k. Substituting 'n' gives:

ileaved = ((rsb * k + i) / k) + ((rsb * k + i) % k) * region_blocks * block_size
= rsb + (i * region_blocks * block_size)

The result is more efficient and easier to understand.

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