xfs: test that mkfs creates zone-aligned RT devices

xfs: test that mkfs creates zone-aligned RT devices

Make sure mkfs doesn't create unmountable file systems and instead rounds
down the RT subvolume size to a multiple of the zone size.

Two passes: one with a device that is not aligned, and one for an
explicitly specified unaligned RT device size.

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