selftests/bpf: Fix incorrect array size calculation

The loop in bench_sockmap_prog_destroy() has two issues:

1. Using 'sizeof(ctx.fds)' as the loop bound results in the number of
   bytes, not the number of file descriptors, causing the loop to iterate
   far more times than intended.

2. The condition 'ctx.fds[0] > 0' incorrectly checks only the first fd for
   all iterations, potentially leaving file descriptors unclosed. Change
   it to 'ctx.fds[i] > 0' to check each fd properly.

These fixes ensure correct cleanup of all file descriptors when the
benchmark exits.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250909124721.191555-1-jiayuan.chen@linux.dev

Closes: https://lore.kernel.org/bpf/aLqfWuRR9R_KTe5e@stanley.mountain/