ipvs: remove extra rcu lock

In 3.7 we added code that uses ipv4_update_pmtu but after commit
c5ae7d41

 (ipv4: must use rcu protection while calling fib_lookup)
the RCU lock is not needed.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>