s390/dump: fix copying to user-space of swapped kdump oldmem

This commit fixes a bug introduced by commit e9e7870f ("s390/dump:
introduce boot data 'oldmem_data'").
OLDMEM_BASE was mistakenly replaced by oldmem_data.size instead of
oldmem_data.start.

This bug caused the following error during kdump:
kdump.sh[878]: No program header covering vaddr 0x3434f5245found kexec bug?

Fixes: e9e7870f ("s390/dump: introduce boot data 'oldmem_data'")
Cc: stable@vger.kernel.org # 5.15+
Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Reviewed-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/kernel/crash_dump.c

@@ -191,8 +191,8 @@ static int copy_oldmem_user(void __user *dst, void *src, size_t count)
 				return rc;
 		} else {
 			/* Check for swapped kdump oldmem areas */
-			if (oldmem_data.start && from - oldmem_data.size < oldmem_data.size) {
-				from -= oldmem_data.size;
+			if (oldmem_data.start && from - oldmem_data.start < oldmem_data.size) {
+				from -= oldmem_data.start;
 				len = min(count, oldmem_data.size - from);
 			} else if (oldmem_data.start && from < oldmem_data.size) {
 				len = min(count, oldmem_data.size - from);