xfrm: iptfs: fix stack OOB read in iptfs_skb_reset_frag_walk()

In the Linux kernel, the following vulnerability has been resolved:

xfrm: iptfs: fix stack OOB read in iptfs_skb_reset_frag_walk()

iptfs_skb_reset_frag_walk() advances to the fragment containing @offset
with an unbounded loop:

while (offset >= walk->past + walk->frags[walk->fragi].len)
walk->past += walk->frags[walk->fragi++].len;

walk->fragi is advanced and walk->frags[walk->fragi] is dereferenced
without ever checking fragi against walk->nr_frags. When the requested
offset is at or beyond the total length spanned by the walk's fragments,
fragi runs past nr_frags and off the end of the fixed-size on-stack
frags[MAX_SKB_FRAGS + 1] array, reading out-of-bounds stack memory.

The two callers behave differently: iptfs_skb_add_frags() already guards
against this with

if (!walk->nr_frags ||
offset >= walk->total + walk->initial_offset)
return len;

but iptfs_skb_can_add_frags() has no such guard and calls
iptfs_skb_reset_frag_walk() unconditionally, so it performs the
out-of-range walk. Its own "fragi < walk->nr_frags" bound check runs only
afterwards, too late to prevent the read.

This is reachable from the receive path: a crafted IP-TFS (AGGFRAG)
payload delivered to an IPTFS SA drives iptfs_reassem_cont() ->
iptfs_skb_can_add_frags() with an offset past the fragment total, e.g.:

BUG: KASAN: stack-out-of-bounds in iptfs_skb_reset_frag_walk+0x235/0x250
Read of size 4 at addr ffff888008ad7210 by task repro/345
iptfs_skb_reset_frag_walk+0x235/0x250 net/xfrm/xfrm_iptfs.c:392
iptfs_skb_can_add_frags+0x155/0x310 net/xfrm/xfrm_iptfs.c:420
iptfs_reassem_cont+0xcf8/0x1140 net/xfrm/xfrm_iptfs.c:902
iptfs_input_ordered+0x552/0x670 net/xfrm/xfrm_iptfs.c:1280
iptfs_input+0x3d6/0xde0 net/xfrm/xfrm_iptfs.c:1741
xfrm_input+0x282f/0x6140 net/xfrm/xfrm_input.c:700
xfrm4_esp_rcv+0x93/0x120 net/ipv4/xfrm4_protocol.c:104
ip_rcv+0x278/0x2d0 net/ipv4/ip_input.c:612

Give iptfs_skb_can_add_frags() the same up-front guard that
iptfs_skb_add_frags() already has, so the walk is never entered with an
out-of-range offset. When it triggers, the caller falls back to the
existing linearize-and-copy path, which is safe.