gro: inline tcp6_gro_receive()

gro: inline tcp6_gro_receive()

FDO/LTO are unable to inline tcp6_gro_receive() from ipv6_gro_receive()

Make sure tcp6_check_fraglist_gro() is only called only when needed,
so that compiler can leave it out-of-line.

$ scripts/bloat-o-meter -t vmlinux.1 vmlinux.2
add/remove: 2/0 grow/shrink: 3/1 up/down: 1123/-253 (870)
Function                                     old     new   delta
ipv6_gro_receive                            1069    1846    +777
tcp6_check_fraglist_gro                        -     272    +272
ipv6_offload_init                            218     274     +56
__pfx_tcp6_check_fraglist_gro                  -      16     +16
ipv6_gro_complete                            433     435      +2
tcp6_gro_receive                             959     706    -253
Total: Before=22592662, After=22593532, chg +0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260120164903.1912995-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>