xfs: map multiple bmbt records to a single rmapbt record

xfs: map multiple bmbt records to a single rmapbt record

Make sure that we can handle multiple bmbt records mapping to a
single rmapbt record.  This can happen if you fallocate more than
2^21 contiguous blocks to a file.

(Also add some helpers that can create huge devices with some
dm-zero and dm-snapshot fakery.)

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