tools/bpf: selftests : Explain bpf_iter test failures with llvm 10.0.0

Commit 6879c042 ("tools/bpf: selftests: Add bpf_iter selftests")
added self tests for bpf_iter feature. But two subtests
ipv6_route and netlink needs llvm latest 10.x release branch
or trunk due to a bug in llvm BPF backend. This patch added
the file README.rst to document these two failures
so people using llvm 10.0.0 can be aware of them.
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200513180215.2949237-1-yhs@fb.com

tools/testing/selftests/bpf/README.rst

+==================
+BPF Selftest Notes
+==================
+
+Additional information about selftest failures are
+documented here.
+
+bpf_iter test failures with clang/llvm 10.0.0
+=============================================
+
+With clang/llvm 10.0.0, the following two bpf_iter tests failed:
+  * ``bpf_iter/ipv6_route``
+  * ``bpf_iter/netlink``
+
+The symptom for ``bpf_iter/ipv6_route`` looks like
+
+.. code-block:: c
+
+  2: (79) r8 = *(u64 *)(r1 +8)
+  ...
+  14: (bf) r2 = r8
+  15: (0f) r2 += r1
+  ; BPF_SEQ_PRINTF(seq, "%pi6 %02x ", &rt->fib6_dst.addr, rt->fib6_dst.plen);
+  16: (7b) *(u64 *)(r8 +64) = r2
+  only read is supported
+
+The symptom for ``bpf_iter/netlink`` looks like
+
+.. code-block:: c
+
+  ; struct netlink_sock *nlk = ctx->sk;
+  2: (79) r7 = *(u64 *)(r1 +8)
+  ...
+  15: (bf) r2 = r7
+  16: (0f) r2 += r1
+  ; BPF_SEQ_PRINTF(seq, "%pK %-3d ", s, s->sk_protocol);
+  17: (7b) *(u64 *)(r7 +0) = r2
+  only read is supported
+
+This is due to a llvm BPF backend bug. The fix 
+  https://reviews.llvm.org/D78466
+has been pushed to llvm 10.x release branch and will be
+available in 10.0.1. The fix is available in llvm 11.0.0 trunk.