mptcp: shrink mptcp_out_options struct

After the previous patch we can alias with a union several
fields in mptcp_out_options. Such struct is stack allocated and
memset() for each plain TCP out packet. Every saved byted counts.

Before:
pahole -EC mptcp_out_options
# ...
/* size: 136, cachelines: 3, members: 17 */

After:
pahole -EC mptcp_out_options
# ...
/* size: 56, cachelines: 1, members: 9 */

Signed-off-by: Paolo Abeni <[e-mail]>
Signed-off-by: Mat Martineau <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/mptcp.h

···
 struct mptcp_out_options {
 #if IS_ENABLED(CONFIG_MPTCP)
 	u16 suboptions;
-	u64 sndr_key;
-	u64 rcvr_key;
-	u64 ahmac;
-	struct mptcp_addr_info addr;
 	struct mptcp_rm_list rm_list;
 	u8 join_id;
 	u8 backup;
···
 	   reset_transient:1,
 	   csum_reqd:1,
 	   allow_join_id0:1;
-	u32 nonce;
-	u64 thmac;
-	u32 token;
-	u8 hmac[20];
-	struct mptcp_ext ext_copy;
+	union {
+		struct {
+			u64 sndr_key;
+			u64 rcvr_key;
+		};
+		struct {
+			struct mptcp_addr_info addr;
+			u64 ahmac;
+		};
+		struct mptcp_ext ext_copy;
+		struct {
+			u32 nonce;
+			u32 token;
+			u64 thmac;
+			u8 hmac[20];
+		};
+	};
 #endif
 };