s390/setup: don't reserve memory that occupied decompressor's head

There is no useful information within [STARTUP_NORMAL_OFFSET, HEAD_END] now.

But the memory region [0, STARTUP_NORMAL_OFFSET] is used by:
* lowcore
* kdump for swapping memory
* stand-alone zipl dumpers for code, data, stack and heap
Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/kernel/setup.c

@@ -797,7 +797,7 @@ static void __init reserve_kernel(void)
 {
 	unsigned long start_pfn = PFN_UP(__pa(_end));
 
-	memblock_reserve(0, HEAD_END);
+	memblock_reserve(0, STARTUP_NORMAL_OFFSET);
 	memblock_reserve((unsigned long)_stext, PFN_PHYS(start_pfn)
 			 - (unsigned long)_stext);
 }