uidgid: add map_id_range_up()

Add map_id_range_up() to verify that the full kernel id range can be
mapped up in a given idmapping. This will be used in follow-up patches.

Link: https://lore.kernel.org/r/20250204-work-mnt_idmap-statmount-v2-1-007720f39f2e@kernel.org


Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>