rtnetlink: fix gcc -Wconversion warning

RTA_ALIGNTO is currently define as 4. It has to be 4U to prevent warning
for RTA_ALIGN and RTA_DATA expansions when -Wconversion gcc option is
enabled.
This follows NLMSG_ALIGNTO definition in <include/uapi/linux/netlink.h>.
Signed-off-by: Ronen Arad <ronen.arad@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/rtnetlink.h

@@ -160,7 +160,7 @@ struct rtattr {
 
 /* Macros to handle rtattributes */
 
-#define RTA_ALIGNTO	4
+#define RTA_ALIGNTO	4U
 #define RTA_ALIGN(len) ( ((len)+RTA_ALIGNTO-1) & ~(RTA_ALIGNTO-1) )
 #define RTA_OK(rta,len) ((len) >= (int)sizeof(struct rtattr) && \
 			 (rta)->rta_len >= sizeof(struct rtattr) && \