xfs: directory scrub should check the null bestfree entries too

Teach the directory scrubber to check all the bestfree entries,
including the null ones.  We want to be able to detect the case where
the entry is null but there actually /is/ a directory data block.

Found by fuzzing lbests[0] = ones in xfs/391.

Fixes: df481968 ("xfs: scrub directory freespace")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>