s390/crash_dump: fix virtual vs physical address handling

Signal processor STORE STATUS requires a physical address where register
contents are supposed to be written to, however the kernel must read the
data via the corresponding virtual address.

Also the allocated save_area, where register contents are copied to,
resides in virtual address space.

Fix this by using proper __pa() conversion, or correct memblock_alloc()
invocation.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>