bpf: re-fix skip write only files in debugfs

bpf: re-fix skip write only files in debugfs

Commit 5bc60de50dfe ("selftests: bpf: Don't try to read files without
read permission") got reverted as the fix was not working as expected
and real fix came in via 8101e069418d ("selftests: bpf: Skip write
only files in debugfs"). When bpf-next got merged into net-next, the
test_offload.py had a small conflict. Fix the resolution in ae8a76fb8b5d
iby not reintroducing 5bc60de50dfe again.

Fixes: ae8a76fb8b5d ("Merge git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jakub Kicinski <jakub.kicinski@netronome.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>