net: FIX ipv6_forward sysctl restart

Just returning -ERESTARTSYS without a signal pending is not
good that will just leak it to userspace.  We need return
-ERESTARTNOINTR so we always restart and set signal pending
so that we fall of the fast path of syscall return and setup
the system call restart.

So use restart_syscall() which does all of this for us.
Signed-off-by: NEric W. Biederman <ebiederm@aristanetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -503,7 +503,7 @@ static int addrconf_fixup_forwarding(struct ctl_table *table, int *p, int old)
 		return 0;
 
 	if (!rtnl_trylock())
-		return -ERESTARTSYS;
+		return restart_syscall();
 
 	if (p == &net->ipv6.devconf_all->forwarding) {
 		__s32 newf = net->ipv6.devconf_all->forwarding;