mkfs: set required parts of the realtime geometry before computing log geometry

mkfs: set required parts of the realtime geometry before computing log geometry

The minimum log size depends on the transaction reservation sizes, which
in turn depend on the realtime device geometry.  Therefore, we need to
set up some of the rt geometry before we can compute the real minimum
log size.

This fixes a problem where mkfs, given a small data device and a
realtime volume, formats a filesystem with a log that is too small to
pass the mount time log size checks.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>