Fix yet more warnings from gcc 4.6:

block/block_compact.c:193:165: error: conversion to 'long unsigned int' from 'off_t' may change the sign of the result [-Werror=sign-conversion]
block/block_compact.c:203:170: error: conversion to 'long unsigned int' from 'off_t' may change the sign of the result [-Werror=sign-conversion]