bcachefs: Make sure opts.read_only gets propagated back to VFS

If we think we're read-only but the VFS doesn't, fun will ensue.

And now that we know we have to be able to do this safely, just make
nochanges imply ro.

Reported-by:  <syzbot+a7d6ceaba099cc21dee4@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>