[TCP]: Fix MD5 signature handling on big-endian.

Based upon a report and initial patch by Peter Lieven.

tcp4_md5sig_key and tcp6_md5sig_key need to start with
the exact same members as tcp_md5sig_key.  Because they
are both cast to that type by tcp_v{4,6}_md5_do_lookup().

Unfortunately tcp{4,6}_md5sig_key use a u16 for the key
length instead of a u8, which is what tcp_md5sig_key
uses.  This just so happens to work by accident on
little-endian, but on big-endian it doesn't.

Instead of casting, just place tcp_md5sig_key as the first member of
the address-family specific structures, adjust the access sites, and
kill off the ugly casts.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 185c7ec..54053de 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1059,14 +1059,12 @@
 };
 
 struct tcp4_md5sig_key {
-	u8			*key;
-	u16			keylen;
+	struct tcp_md5sig_key	base;
 	__be32			addr;
 };
 
 struct tcp6_md5sig_key {
-	u8			*key;
-	u16			keylen;
+	struct tcp_md5sig_key	base;
 #if 0
 	u32			scope_id;	/* XXX */
 #endif