mds: start resolve with root as UNKNOWN (if it's not ours)

mds: start resolve with root as UNKNOWN (if it's not ours)

Anything that's not ours should be unknown, including the root dir frag,
which normally starts out as mds0.

If we leave it as 0, then when mds0 claims a subset of /, its bounds are
left as 0 as well instead of being set to unknown.  Which leads to
incorrect resolve stage results.