ip6mr: do not acquire mrt_lock in pim6_rcv()

rcu_read_lock() protection is more than enough.

vif_dev_read() supports either mrt_lock or rcu_read_lock().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>