xfs: refactor the directory data block bestfree checks

In a directory data block, the zeroth bestfree item must point to the
longest free space.  Therefore, when we check the bestfree block's
records against the data blocks, we only need to compare with bf[0] and
don't need the loop.

The weird loop was most probably the result of an earlier refactoring
gone bad.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>