Fix integer overflow error in bcache_init()
Fix integer overflow error in bcache_init()

- When buffer cache size is bigger than 2GB, allocating a big blob
during the initialization of buffer cache fails due to integer
overflow.

Change-Id: I7d5e2ef05808d40e56f4d7fbe080f8c4a8837724

wurikiji/forestdb,hisundar/forestdb,couchbase/forestdb,wurikiji/forestdb,hisundar/forestdb,couchbase/forestdb