[S390] Fix memory detection.

Before we're getting short on memory detection fixes here is the next
one: if neither sclp nor diag260 report the storage size the detection
loop will return immediately without detecting anything. Fix this by
breaking the detection loop only if the memory end is known.
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/early.c

@@ -200,7 +200,7 @@ static noinline __init void find_memory_chunks(unsigned long memsize)
 		cc = __tprot(addr);
 		while (cc == old_cc) {
 			addr += CHUNK_INCR;
-			if (addr >= memsize)
+			if (memsize && addr >= memsize)
 				break;
 #ifndef CONFIG_64BIT
 			if (addr == ADDR2G)