bpf: add test for the verifier equal logic bug

This is a test to verify that

bpf: fix states equal logic for varlen access

actually fixed the problem.  The problem was if the register we added to our map
register was UNKNOWN in both the false and true branches and the only thing that
changed was the range then we'd incorrectly assume that the true branch was
valid, which it really wasnt.  This tests this case and properly fails without
my fix in place and passes with it in place.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>