fix compiler warning in object_cache.c

When building sheepdog it reports warning message like:

	object_cache.c: In function ‘object_cache_flush_and_delete’:
	object_cache.c:1039: warning: comparison is always false due to limited range of data type
	object_cache.c: In function ‘load_cache_object’:
	object_cache.c:1252: warning: comparison is always false due to limited range of data type
	object_cache.c: In function ‘load_cache’:
	object_cache.c:1290: warning: comparison is always false due to limited range of data type

The 'idx' is the result of strtoul and it should be 'unsgiend long' instead of 'uint32_t' and
it can't reach ULLONG_MAX but ULONG_MAX.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>