wipe_lv: use BLKZEROOUT when possible

wipe_lv: use BLKZEROOUT when possible

Since BLKZEROOUT ioctl should be supposedly fastest
way how to clear block device start using this ioctl
for zeroing a device. Commonly we do zero typically
small portion of a device (8KiB) - however since we now
also started to zero metadata devices, in the case
of i.e. thin-pool metadata this can go upto ~16GiB
and here the performance starts to be noticable.