net: add pskb_may_pull() to skb_gro_receive_list()

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

net: add pskb_may_pull() to skb_gro_receive_list()

skb_gro_receive_list() calls skb_pull(skb, skb_gro_offset(skb)) without

first ensuring the data is in the linear area via pskb_may_pull(). When

the skb arrives via napi_gro_frags(), skb_headlen can be 0 (all data in

page fragments) while skb_gro_offset is non-zero (after IP+TCP header

parsing). The skb_pull() then decrements skb->len by skb_gro_offset

but skb->data_len stays unchanged, hitting BUG_ON(skb->len < skb->data_len)

in __skb_pull().

The UDP fraglist GRO path already contains this guard at

udp_offload.c:749. Adding it to skb_gro_receive_list() itself provides

centralized protection for all callers (TCP, UDP, and any future

protocols), and ensures the precondition of skb_pull() is satisfied

before it is called.

On pskb_may_pull() failure, set NAPI_GRO_CB(skb)->flush = 1 so the

skb is not held as a new GRO head and is instead delivered through the

normal receive path, matching the UDP handling.