Revert "s390/mm: get rid of VMEM_MAX_PHYS macro"

This reverts commit 456be42a.

The assumption VMEM_MAX_PHYS should match ident_map_size
is wrong. At least discontiguous saved segments (DCSS)
could be loaded at addresses beyond ident_map_size and
dcssblk device driver might fail as result.

Reported-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>