New getsockopt option to get socket cookie

Introduce a new getsockopt operation to retrieve the socket cookie
for a specific socket based on the socket fd.  It returns a unique
non-decreasing cookie for each socket.
Tested: https://android-review.googlesource.com/#/c/358163/

Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Chenbo Feng <fengc@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/mips/include/uapi/asm/socket.h

@@ -114,4 +114,6 @@
 
 #define SO_INCOMING_NAPI_ID	56
 
+#define SO_COOKIE		57
+
 #endif /* _UAPI_ASM_SOCKET_H */