x86/irq: Get rid of the 'first_system_vector' indirection bogosity

This variable is beyond pointless. Nothing allocates a vector via
alloc_gate() below FIRST_SYSTEM_VECTOR. So nothing can change
first_system_vector.

If there is a need for a gate below FIRST_SYSTEM_VECTOR then it can be
added to the vector defines and FIRST_SYSTEM_VECTOR can be adjusted
accordingly.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/20170828064956.357109735@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index d0a21b1..a7f36ab 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -5,6 +5,7 @@
 #include <asm/ldt.h>
 #include <asm/mmu.h>
 #include <asm/fixmap.h>
+#include <asm/irq_vectors.h>
 
 #include <linux/smp.h>
 #include <linux/percpu.h>
@@ -482,16 +483,14 @@ static inline void _set_gate(int gate, unsigned type, void *addr,
 				0, 0, __KERNEL_CS);			\
 	} while (0)
 
-extern int first_system_vector;
 /* used_vectors is BITMAP for irq is not managed by percpu vector_irq */
 extern unsigned long used_vectors[];
 
 static inline void alloc_system_vector(int vector)
 {
+	BUG_ON(vector < FIRST_SYSTEM_VECTOR);
 	if (!test_bit(vector, used_vectors)) {
 		set_bit(vector, used_vectors);
-		if (first_system_vector > vector)
-			first_system_vector = vector;
 	} else {
 		BUG();
 	}