ipv6: optimize fl6_update_dst()

ipv6: optimize fl6_update_dst()

fl6_update_dst() is called for every TCP (and others) transmit,
and is a nop for common cases.

Split it in two parts :

1) fl6_update_dst() inline helper, small and fast.

2) __fl6_update_dst() for the exception, out of line.

Small size increase to get better TX performance.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 2/2 grow/shrink: 8/0 up/down: 296/-125 (171)
Function                                     old     new   delta
__fl6_update_dst                               -     104    +104
rawv6_sendmsg                               2244    2284     +40
udpv6_sendmsg                               3013    3043     +30
tcp_v6_connect                              1514    1534     +20
cookie_v6_check                             1501    1519     +18
ip6_datagram_dst_update                      673     690     +17
inet6_sk_rebuild_header                      499     516     +17
inet6_csk_route_socket                       507     524     +17
inet6_csk_route_req                          343     360     +17
__pfx___fl6_update_dst                         -      16     +16
__pfx_fl6_update_dst                          16       -     -16
fl6_update_dst                               109       -    -109
Total: Before=22570304, After=22570475, chg +0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260128185548.3738781-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>