reflink: fix fragmentation tests to work on >4k block size filesystems

reflink: fix fragmentation tests to work on >4k block size filesystems

For tests that only overwrite part of a file, only consider the number
of extents in the *rewritten* blocks when deciding if the FS
fragmentation performance is satisfactory.

(Also slip in a fix for xfs/127 so that it formats correctly when we
specify big blocksizes via MKFS_OPTIONS.)

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>