selftests/bpf: Validate frozen map contents stays frozen

Test that frozen and mmap()'ed BPF map can't be mprotect()'ed as writable or
executable memory. Also validate that "downgrading" from writable to read-only
doesn't screw up internal writable count accounting for the purposes of map
freezing.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200410202613.3679837-2-andriin@fb.com