gro: inline tcp6_gro_complete()

gro: inline tcp6_gro_complete()

Remove one function call from GRO stack for native IPv6 + TCP packets.

$ scripts/bloat-o-meter -t vmlinux.2 vmlinux.3
add/remove: 0/0 grow/shrink: 1/1 up/down: 298/-5 (293)
Function                                     old     new   delta
ipv6_gro_complete                            435     733    +298
tcp6_gro_complete                            311     306      -5
Total: Before=22593532, After=22593825, chg +0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260120164903.1912995-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>