xfs: allow internal RT devices for zoned mode

xfs: allow internal RT devices for zoned mode

Source kernel commit: bdc03eb5f98f6f1ae4bd5e020d1582a23efb7799

Allow creating an RT subvolume on the same device as the main data
device.  This is mostly used for SMR HDDs where the conventional zones
are used for the data device and the sequential write required zones
for the zoned RT section.

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