bpf: Prevent writable memory-mapping of read-only ringbuf pages

Only the very first page of BPF ringbuf that contains consumer position
counter is supposed to be mapped as writeable by user-space. Producer
position is read-only and can be modified only by the kernel code. BPF ringbuf
data pages are read-only as well and are not meant to be modified by
user-code to maintain integrity of per-record headers.

This patch allows to map only consumer position page as writeable and
everything else is restricted to be read-only. remap_vmalloc_range()
internally adds VM_DONTEXPAND, so all the established memory mappings can't be
extended, which prevents any future violations through mremap()'ing.

Fixes: 457f4436 ("bpf: Implement BPF ring buffer and verifier support for it")
Reported-by: Ryota Shiga (Flatt Security)
Reported-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>