user-util,storagectl: introduce USERNS_RANGE_SIZE macro

The mount.storage helper open-codes the conventional 64K UID/GID
delegation block size as 0x10000 / 0x10000U in four places. Several
other places in the tree do the same (nspawn's arg_uid_range default,
homed's mount setup, …), but with no shared name.

Add USERNS_RANGE_SIZE in user-util.h alongside UID_NOBODY and friends,
and switch storagectl over to it. Other call sites can adopt it
incrementally.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

src/shared/nsresource.h

@@ -2,9 +2,10 @@
 #pragma once
 
 #include "shared-forward.h"
+#include "user-util.h"
 
 /* Helpful constants for the only numbers of UIDs that can currently be allocated */
-#define NSRESOURCE_UIDS_64K 0x10000U
+#define NSRESOURCE_UIDS_64K USERNS_RANGE_SIZE
 #define NSRESOURCE_UIDS_1 1U
 
 int nsresource_connect(sd_varlink **ret);