samples/bpf: Use %lu format specifier for unsigned long values

Currently %ld format specifiers are being used for unsigned long
values. Fix this by using %lu instead. Cleans up cppcheck warnings:

warning: %ld in format string (no. 1) requires 'long' but the argument
type is 'unsigned long'. [invalidPrintfArgType_sint]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/bpf/20231219152307.368921-1-colin.i.king@gmail.com