fs: port HAS_UNMAPPED_ID() to vfs{g,u}id_t

The HAS_UNMAPPED_ID() helper is fully self contained so we can port it
to vfs{g,u}id_t without much effort.

Cc: Seth Forshee <sforshee@digitalocean.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>