net: release reference to inet6_dev pointer

addrconf_prefix_rcv returned early without releasing the inet6_dev
pointer when the PIO lifetime is less than accept_ra_min_lft.

Fixes: 5027d54a ("net: change accept_ra_min_rtr_lft to affect all RA lifetimes")
Cc: Maciej Żenczykowski <maze@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Cc: David Ahern <dsahern@kernel.org>
Cc: Simon Horman <horms@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Patrick Rohr <prohr@google.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -2742,7 +2742,7 @@ void addrconf_prefix_rcv(struct net_device *dev, u8 *opt, int len, bool sllao)
 	}
 
 	if (valid_lft != 0 && valid_lft < in6_dev->cnf.accept_ra_min_lft)
-		return;
+		goto put;
 
 	/*
 	 *	Two things going on here: