sys/kern: Retry nlookup if nresolve returned ESTALE

sys/kern: Retry nlookup if nresolve returned ESTALE

Make nlookup retry the whole thing if nresolve VOP returned ESTALE.

This will be used by autofs (in the next few commits).
This doesn't affect other filesystems, because no other filesystem
returns ESTALE.

Autofs needs this because a target filesystem may get mounted under
the root directory of autofs while running nresolve VOP of autofs.
Autofs tells nlookup the topology has changed by returning ESTALE.
Unless nlookup retries the lookup, the result is obsolete.