[TCP] inet_twdr_hangman: Delete unnecessary memory barrier().

As per Ralf Baechle's observations, the schedule_work() call
should give enough of a memory barrier, so the explicit one
here is totally unnecessary.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/inet_timewait_sock.c

@@ -178,7 +178,6 @@ void inet_twdr_hangman(unsigned long data)
 	need_timer = 0;
 	if (inet_twdr_do_twkill_work(twdr, twdr->slot)) {
 		twdr->thread_slots |= (1 << twdr->slot);
-		smp_mb();
 		schedule_work(&twdr->twkill_work);
 		need_timer = 1;
 	} else {