net: add vlan_get_protocol_offset_inline() helper

skb_protocol() is bloated, and forces slow stack canaries in many
fast paths.

Add vlan_get_protocol_offset_inline() which deals with the non-vlan
common cases.

__vlan_get_protocol_offset() is now out of line.

It returns a vlan_type_depth struct to avoid stack canaries in callers.

struct vlan_type_depth {
       __be16 type;
       u16 depth;
};

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/2 grow/shrink: 0/22 up/down: 0/-6320 (-6320)
Function                                     old     new   delta
vlan_get_protocol_dgram                       61      59      -2
__pfx_skb_protocol                            16       -     -16
__vlan_get_protocol_offset                   307     273     -34
tap_get_user                                1374    1207    -167
ip_md_tunnel_xmit                           1625    1452    -173
tap_sendmsg                                  940     753    -187
netif_skb_features                          1079     866    -213
netem_enqueue                               3017    2800    -217
vlan_parse_protocol                          271      50    -221
tso_start                                    567     344    -223
fq_dequeue                                  1908    1685    -223
skb_network_protocol                         434     205    -229
ip6_tnl_xmit                                2639    2409    -230
br_dev_queue_push_xmit                       474     236    -238
skb_protocol                                 258       -    -258
packet_parse_headers                         621     357    -264
__ip6_tnl_rcv                               1306    1039    -267
skb_csum_hwoffload_help                      515     224    -291
ip_tunnel_xmit                              2635    2339    -296
sch_frag_xmit_hook                          1582    1233    -349
bpf_skb_ecn_set_ce                           868     457    -411
IP6_ECN_decapsulate                         1297     768    -529
ip_tunnel_rcv                               2121    1489    -632
ipip6_rcv                                   2572    1922    -650
Total: Before=24892803, After=24886483, chg -0.03%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260204053023.1622775-1-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>