dm-verity-fec: rename block_offset to out_pos

dm-verity-fec: rename block_offset to out_pos

The current position in the output block buffer is called 'pos' in
fec_decode_rsb(), and 'block_offset' in fec_read_bufs() and
fec_decode_bufs().  These names aren't very clear, especially
'block_offset' which is easily confused with the offset of a message or
parity block or the position in the current parity block.

Rename it to 'out_pos'.

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