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: Eric W. Biederman <ebiederm@aristanetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>