xfs: move the btree stats offset into struct btree_ops

The statistics offset is completely static, move it into the btree_ops
structure instead of the cursor.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>