xfs: new EOF fragmentation tests

xfs: new EOF fragmentation tests

These tests create substantial file fragmentation as a result of
application actions that defeat post-EOF preallocation
optimisations. They are intended to replicate known vectors for
these problems, and provide a check that the fragmentation levels
have been controlled. The mitigations we make may not completely
remove fragmentation (e.g. they may demonstrate speculative delalloc
related extent size growth) so the checks don't assume we'll end up
with perfect layouts and hence check for an exceptable level of
fragmentation rather than none.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
[move to different test number, update to current xfstest APIs]
Signed-off-by: Christoph Hellwig <hch@lst.de>