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/basic/user-util.h

@@ -90,6 +90,10 @@ int take_etc_passwd_lock(const char *root);
 #define UID_NOBODY ((uid_t) 65534U)
 #define GID_NOBODY ((gid_t) 65534U)
 
+/* Conventional size of a user-namespace UID/GID delegation block (64K).
+ * Untyped so it can be used in both UID and GID contexts without casts. */
+#define USERNS_RANGE_SIZE 0x10000U
+
 /* If REMOUNT_IDMAPPING_HOST_ROOT is set for remount_idmap() we'll include a mapping here that maps the host
  * root user accessing the idmapped mount to the this user ID on the backing fs. This is the last valid UID in
  * the *signed* 32-bit range. You might wonder why precisely use this specific UID for this purpose? Well, we