sysv_sem: fix the loop that compacts sem array on semaphores removal.

[kib]

Summary

As written now, it copies random kernel memory from beyond the bounds of the array.

While there, use designated initialisers for seminfo, and add assert in sem_remove() that sema is sane. [These will be separate commits].

Reported and tested by: pho

head/sys/kern/sysv_sem.c

 #define U8_C(x)  x ## U
 #define S16_C(x) x
 #define U16_C(x) x ## U
 #define S32_C(x) x
 #define U32_C(x) x ## U
 #define S64_C(x) x ## LL
 #define U64_C(x) x ## ULL
 
+/*
+ * BUILD_BUG_ON() can happen inside functions where _Static_assert() does not
+ * seem to work.  Use old-schoold-ish CTASSERT from before commit
+ * a3085588a88fa58eb5b1eaae471999e1995a29cf but also make sure we do not
+ * end up with an unused typedef or variable. The compiler should optimise
+ * it away entirely.
+ */
+#define _O_CTASSERT(x)          _O__CTASSERT(x, __LINE__)
+#define _O__CTASSERT(x, y)      _O___CTASSERT(x, y)
+#define _O___CTASSERT(x, y)     while (0) { \
+    typedef char __assert_line_ ## y[(x) ? 1 : -1]; \
+    __assert_line_ ## y _x; \
+    _x[0] = '\0'; \
+}
+
 #define BUILD_BUG()                     do { CTASSERT(0); } while (0)
-#define BUILD_BUG_ON(x)                 CTASSERT(!(x))
+#define BUILD_BUG_ON(x)                 _O_CTASSERT(!(x))
 #define BUILD_BUG_ON_MSG(x, msg)        BUILD_BUG_ON(x)
 #define BUILD_BUG_ON_NOT_POWER_OF_2(x)  BUILD_BUG_ON(!powerof2(x))
 #define BUILD_BUG_ON_INVALID(expr)      while (0) { (void)(expr); }
 
 extern const volatile int lkpi_build_bug_on_zero;
 #define BUILD_BUG_ON_ZERO(x)    ((x) ? lkpi_build_bug_on_zero : 0)
 
 #define BUG()                   panic("BUG at %s:%d", __FILE__, __LINE__)