selftests: bpf: Don't try to read files without read permission

Recently couple of files that are write only were added to netdevsim
debugfs. Don't read these files and avoid error.
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>

tools/testing/selftests/bpf/test_offload.py

@@ -312,7 +312,7 @@ class DebugfsDir:
             if f == "ports":
                 continue
             p = os.path.join(path, f)
-            if os.path.isfile(p):
+            if os.path.isfile(p) and os.access(p, os.R_OK):
                 _, out = cmd('cat %s/%s' % (path, f))
                 dfs[f] = out.strip()
             elif os.path.isdir(p):