cache: tidy up handling of foo/bar lookups where foo is not a directory

cache: tidy up handling of foo/bar lookups where foo is not a directory

The code was performing an avoidable check for doomed state to account
for foo being a VDIR but turning VBAD. Now that dooming puts a vnode
in a permanent "modify" state this is no longer necessary as the final
status check will catch it.