selftests/bpf: test_progs: Don't leak server_fd in tcp_rtt

server_fd needs to be closed if pthread can't be created.

Fixes: 8a03222f

 ("selftests/bpf: test_progs: fix client/server race in tcp_rtt")
Signed-off-by: Brian Vazquez <brianvv@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20191001173728.149786-2-brianvv@google.com