selftests/bpf: Test file_pos field in bpf_sysctl ctx

Test access to file_pos field of bpf_sysctl context, both read (incl.
narrow read) and write.

  # ./test_sysctl
  ...
  Test case: ctx:file_pos sysctl:read read ok .. [PASS]
  Test case: ctx:file_pos sysctl:read read ok narrow .. [PASS]
  Test case: ctx:file_pos sysctl:read write ok .. [PASS]
  ...

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>