LinuxKPI: Appease -Wunused-but-set-variable warnings from GCC.

- Mark assert dummy variables as __unused.

- Use a dummy (void) cast of the flags argument passed to
  spin_unlock_irqrestore so it gets treated as used.

Reviewed by:	manu, hselasky
Differential Revision:	https://reviews.freebsd.org/D39349

sys/compat/linuxkpi/common/include/linux/kernel.h

@@ -103,7 +103,7 @@
 #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]; \
+    typedef char __unused __assert_line_ ## y[(x) ? 1 : -1]; \
     __assert_line_ ## y _x; \
     _x[0] = '\0'; \
 }

sys/compat/linuxkpi/common/include/linux/spinlock.h

@@ -128,6 +128,7 @@ typedef struct {
 } while (0)
 
 #define	spin_unlock_irqrestore(_l, flags) do {		\
+	(void)(flags);					\
 	spin_unlock(_l);				\
 } while (0)