netlink: fix gcc -Wconversion compilation warning

$ cat << EOF | gcc -Wconversion -xc -S -o/dev/null -
unsigned f(void) {return NLMSG_HDRLEN;}
EOF
<stdin>: In function 'f':
<stdin>:3:26: warning: negative integer implicitly converted to unsigned type
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netlink.h

@@ -70,7 +70,7 @@ struct nlmsghdr {
    Check		NLM_F_EXCL
  */
 
-#define NLMSG_ALIGNTO	4
+#define NLMSG_ALIGNTO	4U
 #define NLMSG_ALIGN(len) ( ((len)+NLMSG_ALIGNTO-1) & ~(NLMSG_ALIGNTO-1) )
 #define NLMSG_HDRLEN	 ((int) NLMSG_ALIGN(sizeof(struct nlmsghdr)))
 #define NLMSG_LENGTH(len) ((len)+NLMSG_ALIGN(NLMSG_HDRLEN))