selftests: net: tcp_mmap should create detached threads

Since we do not plan using pthread_join() in the server do_accept()
loop, we better create detached threads, or risk increasing memory
footprint over time.

Fixes: 192dc405 ("selftests: net: add tcp_mmap program")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>