ipv6: frags: cleanup __IP6_INC_STATS() confusion

ipv6: frags: cleanup __IP6_INC_STATS() confusion

After commits e1ae5c2ea478 ("vrf: Increment Icmp6InMsgs on the original
netdev") and bdb7cc643fc9 ("ipv6: Count interface receive statistics
on the ingress netdev") net/ipv6/reassembly.c uses three different
ways to reach idev in various __IP6_INC_STATS() calls.

- ip6_dst_idev(skb_dst(skb))
- __in6_dev_get_safely(skb->dev)
- __in6_dev_stats_get(skb->dev)

Lets centralize this from ipv6_frag_rcv() and use __in6_dev_stats_get().

Note that ipv6_frag_rcv() tests if skb->dev could be NULL already, so
I chose to also guard against NULL, but we probably can remove the
tests in a followup patch, because I do not think skb->dev could be NULL.

iif = skb->dev ? skb->dev->ifindex : 0;

idev can be NULL, __IP6_INC_STATS() deals with this possibility.

Small code size reduction as a bonus.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-145 (-145)
Function                                     old     new   delta
ipv6_frag_rcv                               2399    2362     -37
ip6_frag_reasm                               705     597    -108
Total: Before=31455552, After=31455407, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260526145529.3587126-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>