xfs: change xfs_iwalk_grab_ichunk to use startino, not lastino

Now that the inode chunk grabbing function is a static function in the
iwalk code, change its behavior so that @agino is the inode where we
want to /start/ the iteration.  This reduces cognitive friction with the
callers and simplifes the code.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>