s390/kdump: Allow copy_oldmem_page() copy to virtual memory

The kdump mmap patch series (git commit 83086978) changed the
requirements for copy_oldmem_page(). Now this function is used for copying
to virtual memory.

So implement vmalloc support for the s390 version of copy_oldmem_page().

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>