In shared_folders mode NewFs derived the shared folder name with
path.Dir(f.root), which returns the parent path rather than the first
path component. For a root like "SharedFolder/subdir/deeper" this yielded
"SharedFolder/subdir", which findSharedFolder cannot match, so NewFs
failed with ErrorDirNotFound. Use the first path component of the root,
as the shared_folders option documents, so deeply nested roots mount.
Fixes#9705