selftests/bpf: Fix array access with signed variable test

The test fails because of a recent fix to the verifier, even though this
program is valid. In details what happens is:

    7: (61) r1 = *(u32 *)(r0 +0)

Load a 32-bit value, with signed bounds [S32_MIN, S32_MAX]. The bounds
of the 64-bit value are [0, U32_MAX]...

    8: (65) if r1 s> 0xffffffff goto pc+1

... therefore this is always true (the operand is sign-extended).

    10: (b4) w2 = 11
    11: (6d) if r2 s> r1 goto pc+1

When true, the 64-bit bounds become [0, 10]. The 32-bit bounds are still
[S32_MIN, 10].

    13: (64) w1 <<= 2

Because this is a 32-bit operation, the verifier propagates the new
32-bit bounds to the 64-bit ones, and the knowledge gained from insn 11
is lost.

    14: (0f) r0 += r1
    15: (7a) *(u64 *)(r0 +0) = 4

Then the verifier considers r0 unbounded here, rejecting the test. To
make the test work, change insn 8 to check the sign of the 32-bit value.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>