xfs: test increased overlong directory extent discard threshold

xfs: test increased overlong directory extent discard threshold

As of 2007, metadump has an interesting "feature" where it discards
directory extents that are longer than 1000 (originally 20) blocks.
This ostensibly was to protect metadump from corrupt bmbt records, but
it also has the effect of omitting from the metadump valid long extents.
The end result is that we create incomplete metadumps, which is
exacerbated by the lack of warning unless -w is passed.

So now that we've fixed the default threshold to MAXEXTLEN, check that
the installed metadump no longer exhibits this behavior.

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