[XFS] fix sparse shadowed variable warnings

- in xfs_probe_cluster rename the inner len to pg_len. There's no harm
  here because the outer len isn't used after the inner len comes into
  existence but it keeps the code clean.
- in xfs_da_do_buf remove the inner i because they don't overlap
  and they are both the same type.

SGI-PV: 968555
SGI-Modid: xfs-linux-melb:xfs-kern:29311a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>