[XFS] kill struct xfs_btree_hdr

This type is only embedded in struct xfs_btree_block and never used
directly. By moving the fields directly into struct xfs_btree_block a lot
of the macros for struct xfs_btree_sblock and struct xfs_btree_lblock can
be used for struct xfs_btree_block too now which helps greatly with some
of the migrations during implementing the generic btree code.

SGI-PV: 985583

SGI-Modid: xfs-linux-melb:xfs-kern:32174a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Bill O'Donnell <billodo@sgi.com>
Signed-off-by: David Chinner <david@fromorbit.com>