lfsr: ensure we don't generate an offset + buflen that exceeds the max size

lfsr: ensure we don't generate an offset + buflen that exceeds the max size

Currently we check for the max value, but that doesn't always
work since it may not fit the minimum block size (even if it
is guaranteed to be smaller than the max offset). Pass in the
last valid block.

Signed-off-by: Jens Axboe <axboe@kernel.dk>