move-only: Move CBlockFileInfo to kernel namespace

Also, move it to the blockstorage module, because it is only used inside
that module.

Can be reviewed with the git option --color-moved=dimmed-zebra

src/test/blockmanager_tests.cpp

@@ -17,6 +17,7 @@
 #include <test/util/logging.h>
 #include <test/util/setup_common.h>
 
+using kernel::CBlockFileInfo;
 using node::STORAGE_HEADER_BYTES;
 using node::BlockManager;
 using node::KernelNotifications;