selftests/bpf: Test ringbuf mmap read-only and read-write restrictions

Extend ringbuf selftest to validate read/write and read-only restrictions on
memory mapping consumer/producer/data pages. Ensure no "escalations" from
PROT_READ to PROT_WRITE/PROT_EXEC is allowed. And test that mremap() fails to
expand mmap()'ed area.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210514180726.843157-1-andrii@kernel.org