dm-verity-fec: simplify computation of rsb

dm-verity-fec: simplify computation of rsb

To compute 'rsb', verity_fec_decode() divides 'offset' by
'v->fec->region_blocks << v->data_dev_block_bits', then subtracts the
quotient times that divisor.  That's simply the long way to do a modulo
operation, i.e. a - b * floor(a / b) instead of just a % b.  Use
div64_u64_rem() to get the remainder more concisely.

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