[XFS] small cleanups in xfs_btree.c

Remove unneeded xfs_btree_get_block forward declaration. Move
xfs_btree_firstrec next to xfs_btree_lastrec.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31787a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>