hammer_bnew
void *hammer_bnew(hammer_mount_t hmp, hammer_off_t off,
hammer_bnew(hmp, next_offset, errorp, &buffer3);
redo = hammer_bnew(hmp, next_offset, &error, &buffer);
undo = hammer_bnew(hmp, next_offset, &error, &buffer);
head = hammer_bnew(hmp, next_offset, &error, &buffer);