ia64: use mem_data to detect nodes' minimal and maximal PFNs

When EFI memory map is traversed to determine the extents of each node, the
minimal and maximal PFNs are stored in the bootmem_data structures. The
same information ls later stored in the mem_data array of 'struct
early_node_data'.

Switch to using mem_data from the very beginning.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>