s390/mem_detect: rely on diag260() if sclp_early_get_memsize() fails

s390/mem_detect: rely on diag260() if sclp_early_get_memsize() fails

[ Upstream commit 5966bd7fef1d7662c1250f23982e7c5a0120aa6e ]

In case sclp_early_get_memsize() fails but diag260() succeeds make sure
some sane value is returned. This error scenario is highly unlikely,
but this change makes system able to boot in such case.

Suggested-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Stable-dep-of: 2db7e7c4b86f ("s390/boot: fix mem_detect extended area allocation")
Signed-off-by: Sasha Levin <sashal@kernel.org>