selftests/bpf: Fix u32 variable compared with less than zero

Variable ret is compared with less than zero even though it was set as u32.
So u32 to int conversion is needed.

Signed-off-by: Kang Minchul <tegongkang@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Björn Töpel <bjorn@kernel.org>
Link: https://lore.kernel.org/r/20221105183656.86077-1-tegongkang@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>