xfs/291: use _scratch_mkfs_sized instead of opencoding the logic

xfs/291: use _scratch_mkfs_sized instead of opencoding the logic

Now that _scratch_mkfs_xfs_sized accepts arbitrary mkfs arguments, this
test doesn't need to open-code the helper just to add the dir block and
log size arguments.  Clean this up, which will also help us to avoid
problems with rtrmap if the rt devices is large enough to cause format
failures with the 133M data device size.

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