Fix some dialyzer errors

riak_kv_vnode:fold_extras was only ever called with the atom 'keys' as
its first argument, so I just renamed the function and removed the
unused parameter. I also removed the catch-all pattern match that would
never match.
Create a one-arity build_filter function

When called with `undefined` as the last argument, build_filter/3 was
ignoring its first argument (it still does for backward compatibility).
Because we were providing it with a dummy 'all' argument, dialyzer was
complaining. Since we know at the call-site whether we're calling the
function with `undefined` or not, we can just use the new one-arity
version.

Fixes #923
Merge pull request #923 from basho/bugfix/923-build-filter-type-spec-fix

kv_vnode calling coverage_filter with 'all'

Reviewed-by: kellymclaughlin