xfs: clean up unused codes at xfs_bulkstat()

There are some unused codes at xfs_bulkstat():

- Variable bp is defined to point to the on-disk inode cluster
  buffer, but it proved to be of no practical help.

- We process the chunks of good inodes which were fetched by iterating
  btree records from an AG.  When processing inodes from each chunk,
  the code recomputing agbno if run into the first inode of a cluster,
  however, the agbno is not being used thereafter.

This patch tries to clean up those things.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>