s390/crash: use static swap buffer for copy_to_user_real()

Currently a temporary page-size buffer is allocated for copying
oldmem to user space. That limits copy_to_user_real() operation
only to stages when virtual memory is available and still makes
it possible to fail while the system is being dumped.

Instead of reallocating single page on each copy_oldmem_page()
iteration use a statically allocated buffer.

This also paves the way for a further memcpy_real() rework where
no swap buffer is needed altogether.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Link: https://lore.kernel.org/r/334ed359680c4d45dd32feb104909f610312ef0f.1658206891.git.agordeev@linux.ibm.com


Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>