[XFS] Remove unused argument to xfs_bmap_finish

The firstblock argument to xfs_bmap_finish is not used by that function.
Remove it and cleanup the code a bit.

Patch provided by Eric Sandeen.

SGI-PV: 960196
SGI-Modid: xfs-linux-melb:xfs-kern:28034a

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>