[block-cache] fix potential file descriptor leak

Encapsulate file descriptor into an object, to ensure that an fd will be
closed properly while exception raised, e.g., the block_cache throws
exception during the block_manager's construction.

thin-provisioning/cache_stream.h

@@ -38,7 +38,7 @@ namespace thin_provisioning {
 		block_address nr_blocks_;
 		block_address cache_blocks_;
 
-		int fd_;
+		file_utils::file_descriptor fd_;
 		validator::ptr v_;
 		std::unique_ptr<block_cache> cache_;