bpf: Fix a segment issue when downgrading gso_size

bpf: Fix a segment issue when downgrading gso_size

Linearize the skb when downgrading gso_size because it may trigger a
BUG_ON() later when the skb is segmented as described in [1,2].

Fixes: 2be7e212d5419 ("bpf: add bpf_skb_adjust_room helper")
Signed-off-by: Fred Li <dracodingfly@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/all/20240626065555.35460-2-dracodingfly@gmail.com
Link: https://lore.kernel.org/all/668d5cf1ec330_1c18c32947@willemb.c.googlers.com.notmuch
Link: https://lore.kernel.org/bpf/20240719024653.77006-1-dracodingfly@gmail.com