tcp: move tcp_v4_early_demux() to net/ipv4/ip_input.c

tcp_v4_early_demux() has a single caller : ip_rcv_finish_core().

Move it to net/ipv4/ip_input.c and mark it static, for possible
compiler/linker optimizations.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260306131130.654991-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>