samples/bpf: Remove unneeded variable

Return value directly instead of taking this in another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cm>
Signed-off-by: Minghao Chi <chi.minghao@zte.com.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211209080051.421844-1-chi.minghao@zte.com.cn