ip6mr: do not get a device reference in pim6_rcv()

pim6_rcv() is called under rcu_read_lock(), there is
no need to use dev_hold()/dev_put() pair.

IPv4 side was handled in commit 55747a0a
("ipmr: __pim_rcv() is called under rcu_read_lock")

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