tcp: move tcp_out_of_memory() to net/ipv4/tcp.c

tcp: move tcp_out_of_memory() to net/ipv4/tcp.c

tcp_out_of_memory() has a single caller: tcp_check_oom().

Following patch will also make sk_memory_allocated()
not anymore visible from <net/sock.h> and <net/tcp.h>

Add const qualifier to sock argument of tcp_out_of_memory()
and tcp_check_oom().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240429134025.1233626-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>