generic/204: use available blocks to determine the number of files to create

generic/204: use available blocks to determine the number of files to create

Use the available block count to compute the number of files we think
we can create, rather than hardcoding a particular size.  This fixes
the ENOSPC failures for xfs filesystems with rmap/reflink support.

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>