selftests/bpf: Drop duplicate max/min definitions

Drop duplicate macros min() and MAX() definitions in prog_tests and use
MIN() or MAX() in sys/param.h instead.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/1ae276da9925c2de59b5bdc93b693b4c243e692e.1649462033.git.geliang.tang@suse.com