xfs: store the btree pointer length in struct xfs_btree_ops

xfs: store the btree pointer length in struct xfs_btree_ops

Source kernel commit: 1a9d26291c68fbb8f8d24f9f694b32223a072745

Make the pointer length an explicit field in the btree operations
structure so that the next patch (which introduces an explicit btree
type enum) doesn't have to play a bunch of awkward games with inferring
the pointer length from the enumeration.

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