selftests/bpf: Fix UBSan complaint about signed __int128 overflow

selftests/bpf: Fix UBSan complaint about signed __int128 overflow

Test is using __int128 variable as unsigned and highest order bit can be
set to 1 after bit shift. Use unsigned __int128 explicitly and prevent
UBSan from complaining.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211124002325.1737739-8-andrii@kernel.org