bpf: Fix compilation warning of selftests

Clang compiler version: 12.0.0
The following warning appears during the selftests/bpf compilation:

prog_tests/send_signal.c:51:3: warning: ignoring return value of ‘write’,
declared with attribute warn_unused_result [-Wunused-result]
   51 |   write(pipe_c2p[1], buf, 1);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~
prog_tests/send_signal.c:54:3: warning: ignoring return value of ‘read’,
declared with attribute warn_unused_result [-Wunused-result]
   54 |   read(pipe_p2c[0], buf, 1);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~
......

prog_tests/stacktrace_build_id_nmi.c:13:2: warning: ignoring return value
of ‘fscanf’,declared with attribute warn_unused_result [-Wunused-resul]
   13 |  fscanf(f, "%llu", &sample_freq);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

test_tcpnotify_user.c:133:2: warning:ignoring return value of ‘system’,
declared with attribute warn_unused_result [-Wunused-result]
  133 |  system(test_script);
      |  ^~~~~~~~~~~~~~~~~~~
test_tcpnotify_user.c:138:2: warning:ignoring return value of ‘system’,
declared with attribute warn_unused_result [-Wunused-result]
  138 |  system(test_script);
      |  ^~~~~~~~~~~~~~~~~~~
test_tcpnotify_user.c:143:2: warning:ignoring return value of ‘system’,
declared with attribute warn_unused_result [-Wunused-result]
  143 |  system(test_script);
      |  ^~~~~~~~~~~~~~~~~~~

Add code that fix compilation warning about ignoring return value and
handles any errors; Check return value of library`s API make the code
more secure.

Signed-off-by: Jianlin Lv <Jianlin.Lv@arm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200806104224.95306-1-Jianlin.Lv@arm.com