random: use lockless method of accessing and updating f->reg_idx

Linus pointed out that there is a much more efficient way of avoiding
the problem that we were trying to address in commit 9dfa7bba:
"fix race in drivers/char/random.c:get_reg()".
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

drivers/char/random.c

@@ -1097,15 +1097,15 @@ static void add_interrupt_bench(cycles_t start)
 static __u32 get_reg(struct fast_pool *f, struct pt_regs *regs)
 {
 	__u32 *ptr = (__u32 *) regs;
-	unsigned long flags;
+	unsigned int idx;
 
 	if (regs == NULL)
 		return 0;
-	local_irq_save(flags);
-	if (f->reg_idx >= sizeof(struct pt_regs) / sizeof(__u32))
-		f->reg_idx = 0;
-	ptr += f->reg_idx++;
-	local_irq_restore(flags);
+	idx = READ_ONCE(f->reg_idx);
+	if (idx >= sizeof(struct pt_regs) / sizeof(__u32))
+		idx = 0;
+	ptr += idx++;
+	WRITE_ONCE(f->reg_idx, idx);
 	return *ptr;
 }