xfs: track bulkstat progress by agino

The bulkstat main loop progress is tracked by the "lastino"
variable, which is a full 64 bit inode. However, the loop actually
works on agno/agino pairs, and so there's a significant disconnect
between the rest of the loop and the main cursor. Convert this to
use the agino, and pass the agino into the chunk formatting function
and convert it too.

This gets rid of the inconsistency in the loop processing, and
finally makes it simple for us to skip inodes at any point in the
loop simply by incrementing the agino cursor.

cc: <stable@vger.kernel.org> # 3.17
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>