dropbox: fix shared folder mount for roots nested more than one level deep

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
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