bpf: fix up test cases with mixed signed/unsigned bounds

Fix the few existing test cases that used mixed signed/unsigned
bounds and switch them only to one flavor. Reason why we need this
is that proper boundaries cannot be derived from mixed tests.

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