lfsr: add lfsr_reset()

This enables us to restart a sequence.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
axmap: clear map->first_free when we reset it

Otherwise it could be left at an invalid value. When we clear the map,
we know the first free bit is 0.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Re-seed random generator when a file is reset

Signed-off-by: Jens Axboe <axboe@kernel.dk>
verify: add new experimental mode that requires no meta data

Should work fine, but we need to account and track trims to know
which blocks NOT to verify (or verify as zero).

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