The current method to compare 64-bit numbers for conditional jump is:

1) Compare the high 32-bit first.

2) If the high 32-bit isn't the same, then goto step 4.

3) Compare the low 32-bit.

4) Check the desired condition.

This method is right for unsigned comparison, but it is buggy for signed
comparison, because it does signed comparison for low 32-bit too.

There is only one sign bit in 64-bit number, that is the MSB in the 64-bit
number, it is wrong to treat low 32-bit as signed number and do the signed
comparison for it.

This patch fixes the bug and adds a testcase in selftests/bpf for such bug.
Signed-off-by: Wang YanQing <udknight@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>