xfrm: iptfs: validate inner IPv4 header length in IPTFS payload

Add validation of the inner IPv4 packet tot_len and ihl fields parsed
from decrypted IPTFS payloads in __input_process_payload(). A crafted
ESP packet containing an inner IPv4 header with tot_len=0 causes an
infinite loop: iplen=0 leads to capturelen=min(0, remaining)=0, so the
data offset never advances and the while(data < tail) loop never
terminates, spinning forever in softirq context.

Reject inner IPv4 packets where tot_len < ihl*4 or ihl*4 < sizeof(struct
iphdr), which catches both the tot_len=0 case and malformed ihl values.
The normal IP stack performs this validation in ip_rcv_core(), but IPTFS
extracts and processes inner packets before they reach that layer.

Reported-by: Roshan Kumar <roshaen09@gmail.com>
Fixes: 6c82d2433671 ("xfrm: iptfs: add basic receive packet (tunnel egress) handling")
Cc: stable@vger.kernel.org
Signed-off-by: Roshan Kumar <roshaen09@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>