selftests/bpf: fix __retval() being always ignored

selftests/bpf: fix __retval() being always ignored

Florian Westphal found a bug in and suggested a fix for test_loader.c
processing of __retval tag. Because of this bug the function
test_loader.c:do_prog_test_run() never executed and all __retval test
tags were ignored.

If this bug is fixed a number of test cases from
progs/verifier_array_access.c fail with retval not matching the
expected value. This test was recently converted to use test_loader.c
and inline assembly in [1]. When doing the conversion I missed the
important detail of test_verifier.c operation: when it creates
fixup_map_array_ro, fixup_map_array_wo and fixup_map_array_small it
populates these maps with a dummy record.

Disabling the __retval checks for the affected verifier_array_access
in this commit to avoid false-postivies in any potential bisects.
The issue is addressed in the next patch.

I verified that the __retval tags are now respected by changing
expected return values for all tests annotated with __retval, and
checking that these tests started to fail.

[1] https://lore.kernel.org/bpf/20230325025524.144043-1-eddyz87@gmail.com/

Fixes: 19a8e06f5f91 ("selftests/bpf: Tests execution support for test_loader.c")
Reported-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/bpf/f4c4aee644425842ee6aa8edf1da68f0a8260e7c.camel@gmail.com/T/
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20230420232317.2181776-3-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>