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.

Fixes: 5f2b6a909574 ("xfrm: iptfs: add skb-fragment sharing code")
Reported-by: Roshan Kumar <roshaen09@gmail.com>
Signed-off-by: Roshan Kumar <roshaen09@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>