s390/vmem: do not silently ignore mapping limit

The only interface that allows drivers establishing
liner mappings is vmem_add_mapping(). It does check
a requested range against allowed limits and a call
to modify_pagetable() with an invalid mapping range
is impossible.

Hence, an attempt to map an address range outside of
the identity mapping or vmemmap array could only be
kernel bug.

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>