net: add pskb_may_pull() to skb_gro_receive_list()

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.

Fixes: 8d95dc474f85 ("net: add code for TCP fraglist GRO")
Reported-by: HanQuan <eilaimemedsnaimel@gmail.com>
Reported-by: MingXuan <bwnie0730@outlook.com>
Signed-off-by: HanQuan <eilaimemedsnaimel@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>