BP_IDENTITY
DVA_SET_OFFSET(BP_IDENTITY(&bp), off);
#define BP_IS_GANG(bp) DVA_GET_GANG(BP_IDENTITY(bp))
#define BP_IS_HOLE(bp) DVA_IS_EMPTY(BP_IDENTITY(bp))
const dva_t *dva = BP_IDENTITY(bp);
DVA_GET_OFFSET(BP_IDENTITY(bp)));
dva = BP_IDENTITY(bp);