net: always inline __skb_incr_checksum_unnecessary()

net: always inline __skb_incr_checksum_unnecessary()

clang does not inline this helper in GRO fast path.

We can save space and cpu cycles.

$ scripts/bloat-o-meter -t vmlinux.0 vmlinux.1
add/remove: 0/2 grow/shrink: 2/0 up/down: 156/-218 (-62)
Function                                     old     new   delta
tcp6_gro_complete                            227     311     +84
tcp4_gro_complete                            325     397     +72
__pfx___skb_incr_checksum_unnecessary         32       -     -32
__skb_incr_checksum_unnecessary              186       -    -186
Total: Before=22592724, After=22592662, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260120164903.1912995-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>