bpf: Clear decap state on skb_adjust_room shrink path

On shrink in bpf_skb_adjust_room(), apply decapsulation state updates
according to BPF_F_ADJ_ROOM_DECAP_* flags.

For GSO skbs, clear only the tunnel gso_type bits that correspond to
the requested decap layer:

- DECAP_L4_UDP: SKB_GSO_UDP_TUNNEL{,_CSUM}
- DECAP_L4_GRE: SKB_GSO_GRE{,_CSUM}
- DECAP_IPXIP4: SKB_GSO_IPXIP4
- DECAP_IPXIP6: SKB_GSO_IPXIP6

Then clear skb->encapsulation only if no tunnel GSO bits remain, keeping
encapsulation set for cases such as ESP-in-UDP where tunnel state remains.

For non-GSO skbs, there are no tunnel GSO bits to consult, so clear
skb->encapsulation directly when DECAP_L4_* or DECAP_IPXIP_* flags are set.

This keeps decap state handling consistent between GSO and non-GSO packets.

Co-developed-by: Max Tottenham <mtottenh@akamai.com>
Co-developed-by: Anna Glasgall <aglasgal@akamai.com>
Signed-off-by: Max Tottenham <mtottenh@akamai.com>
Signed-off-by: Anna Glasgall <aglasgal@akamai.com>
Signed-off-by: Nick Hudson <nhudson@akamai.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/bpf/20260812083115.73100-6-nhudson@akamai.com

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