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>