when ringbuffer map size is not a power-of-2 multiple of page size, prompt users in a user-friendly way

[hengyoush]

example:

struct {
	__uint(type, BPF_MAP_TYPE_RINGBUF);
	__uint(max_entries, 4097);
} rb SEC(".maps");

then output:

program dev_hard_start_xmit: map rb: EINVAL (ring map size 4097 not a power of two page size(pageSize 4096))

[lmb]

I've pushed a small refactoring to reuse an existing implementation of pow we already had in the code base.