s390/extmem: improve reporting of -ERANGE error

Interface segment_warning() reports maximum mappable physical
address for -ERANGE error. Currently that address is the value
of VMEM_MAX_PHYS macro, but that well might change. A better
way to obtain that address is calling arch_get_mappable_range()
callback - one that is used by vmem_add_mapping() and generates
-ERANGE error in the first place.

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