systemd-nspawn: make SettingsMask 64 bit wide

The use of UINT64_C() in the SettingsMask enum definition is misleading:
it does not mean that individual fields have this width. E.g., with
enum {
   FOO = UINT64_C(1)
}
sizeof(FOO) gives 4. It only means that the shift is done properly. So
1 << 35 is undefined, but UINT64_C(1) << 35 is the expected 64 bit
constant. Thus, the use UINT64_C() is useful, because we know that the shifts
are done properly, no matter what the value of _RLIMIT_MAX is, but when those
fields are used in expressions, we don't know what size they will be
(probably 4). Let's add a define which "hides" the enum definition behind a
define which gives the same value but is actually 64 bit. I think this is a
nicer solution than requiring all users to cast SETTING_RLIMIT_FIRST before
use.

Fixes #9035.

src/nspawn/nspawn-settings.h

@@ -56,9 +56,19 @@ typedef enum SettingsMask {
         SETTING_CPU_AFFINITY      = UINT64_C(1) << 20,
         SETTING_RLIMIT_FIRST      = UINT64_C(1) << 21, /* we define one bit per resource limit here */
         SETTING_RLIMIT_LAST       = UINT64_C(1) << (21 + _RLIMIT_MAX - 1),
-        _SETTINGS_MASK_ALL        = (UINT64_C(1) << (21 + _RLIMIT_MAX)) - 1
+        _SETTINGS_MASK_ALL        = (UINT64_C(1) << (21 + _RLIMIT_MAX)) - 1,
+        _FORCE_ENUM_WIDTH         = UINT64_MAX
 } SettingsMask;
 
+/* We want to use SETTING_RLIMIT_FIRST in shifts, so make sure it is really 64 bits
+ * when used in expressions. */
+#define SETTING_RLIMIT_FIRST ((uint64_t) SETTING_RLIMIT_FIRST)
+#define SETTING_RLIMIT_LAST ((uint64_t) SETTING_RLIMIT_LAST)
+
+assert_cc(sizeof(SettingsMask) == 8);
+assert_cc(sizeof(SETTING_RLIMIT_FIRST) == 8);
+assert_cc(sizeof(SETTING_RLIMIT_LAST) == 8);
+
 typedef struct Settings {
         /* [Run] */
         StartMode start_mode;