Use the canonical idiom for sockaddr_storage.

This works out a bit silly/ugly because the bits/ header file has the
wrong name, so I've also changed the map from kernel struct to boolean
to be a map from kernel struct to filename. That not only fixes this,
it's a bit more readable too. (Just yesterday, when I had no real reason
to change it, I was asking myself "why is this a boolean?"!)

Bug: http://b/236042740
Test: treehugger
Change-Id: I3eee25b493ea97d46cc5dc5fde07f7c5e77d2a46
diff --git a/libc/kernel/uapi/linux/mptcp.h b/libc/kernel/uapi/linux/mptcp.h
index f00abae..d97f29b 100644
--- a/libc/kernel/uapi/linux/mptcp.h
+++ b/libc/kernel/uapi/linux/mptcp.h
@@ -163,13 +163,13 @@
     struct sockaddr sa_local;
     struct sockaddr_in sin_local;
     struct sockaddr_in6 sin6_local;
-    struct __kernel_sockaddr_storage ss_local;
+    struct sockaddr_storage ss_local;
   };
   union {
     struct sockaddr sa_remote;
     struct sockaddr_in sin_remote;
     struct sockaddr_in6 sin6_remote;
-    struct __kernel_sockaddr_storage ss_remote;
+    struct sockaddr_storage ss_remote;
   };
 };
 struct mptcp_subflow_info {