common/rc: teach _scratch_mkfs_sized to set a size on an xfs realtime volume

common/rc: teach _scratch_mkfs_sized to set a size on an xfs realtime volume

Generally speaking, tests that call _scratch_mkfs_sized are trying to
constrain a test's run time by formatting a filesystem that's smaller
than the device.  The current helper does this for the scratch device,
but it doesn't do this for the xfs realtime volume.

If fstests has been configured to create files on the realtime device by
default ("-d rtinherit=1) then those tests that want to run with a small
volume size will instead be running with a huge realtime device.  This
makes certain tests take forever to run, so apply the same sizing to the
rt volume if one exists.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>