s390/mem_detect: add get_mem_detect_online_total()

Add a function to get online memory in total. It is supposed to be used
in the decompressor as well as during early kernel startup.
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/include/asm/mem_detect.h

@@ -66,6 +66,17 @@ static inline int __get_mem_detect_block(u32 n, unsigned long *start,
 	     i < mem_detect.count;					\
 	     i++, __get_mem_detect_block(i, p_start, p_end))
 
+static inline unsigned long get_mem_detect_online_total(void)
+{
+	unsigned long start, end, total = 0;
+	int i;
+
+	for_each_mem_detect_block(i, &start, &end)
+		total += end - start;
+
+	return total;
+}
+
 static inline void get_mem_detect_reserved(unsigned long *start,
 					   unsigned long *size)
 {