Add a helper that validates the startblock is valid.  This checks for a
non-zero block on the main device, but skips that check for blocks on
the realtime device.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>