axmap: fix bug in axmap_find_first_free()

If the map is completely full, we could return an invalid value
for the first free bit. So check if we actually found a match,
return failure if we didn't.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
verify: stop on actual number of bytes needed to be verified

If we don't use LFSR or a random map, we don't get told when
to stop by the random generator or map. So track it on the
side, using the same mechanism as do_io().

Signed-off-by: Jens Axboe <axboe@kernel.dk>
axmap: improve axmap_find_first_free()

Add a ret variable, saves a branch on return.

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