ipv6: move ip6_dst_hoplimit() to net/ipv6/ip6_output.c

ipv6: move ip6_dst_hoplimit() to net/ipv6/ip6_output.c

Move ip6_dst_hoplimit() to net/ipv6/ip6_output.c so that compiler
can (auto)inline it from ip6_xmit().

$ scripts/bloat-o-meter -t vmlinux.0 vmlinux.1
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-11 (-11)
Function                                     old     new   delta
ip6_xmit                                    1684    1673     -11
Total: Before=29655407, After=29655396, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260331174722.4128061-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>