[IA64] Pick highest possible saved_max_pfn for crash_dump

Berhhard Walle noted that on his HP rx8640 he ended up with saved_max_pfn
smaller than the highest address of system ram in /proc/iomem and proposed
a patch to base the address on the unrounded and unfiltered EFI memory
map address.  Simon Horman and Magnus Damm suggested that the whole test
be moved earlier in the function.  This is the combination of both of
these patches.

Signed-off-by: Tony Luck <tony.luck@intel.com>