mkfs: use a default sector size that is also suitable for the rtdev

mkfs: use a default sector size that is also suitable for the rtdev

When creating a filesytem where the data device has a sector size
smalle than that of the RT device without further options, mkfs
currently fails with:

mkfs.xfs: error - cannot set blocksize 512 on block device $RTDEV: Invalid argument

This is because XFS sets the sector size based on logical block size
of the data device, but not that of the RT device.  Change the code
so that is uses the larger of the two values.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>