workaround zfs create -V error for unaligned sizes

workaround zfs create -V error for unaligned sizes

fixes the 'cannot create 'nvme/foo': volume size must be a multiple of
volume block size' error by always rounding the size up to the next 1M
boundary. this is a workaround until
https://github.com/zfsonlinux/zfs/issues/8541 is solved.
the current manpage says 128k is the maximum blocksize, but a local test
showed that values up to 1M are allowed. it might be possible to
increase it even further (see f1512ee61).

Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>