[IA64] Fix boot problems when using "mem=" boot parameter.

[IA64] Fix boot problems when using "mem=" boot parameter.

My tiger-4 machine has 16GB memory. Kernel 2.6.8 fails to boot on it
when command line parameter mem=8G, and it also fails when mem being
set to other value, such as 7G, 10G.

Basically, in function efi_memmap_walk, md->num_pages might be decreased
if mem_limit is set, and then at the next time when efi_memmap_walk is
called, trim_top might trim the md again because of IA64_GRANULE_SIZE
alignment, then another md which is beyond mem_limit at the beginning
will be chosen, and its physical page number is larger than max_pfn. Then,
a BUG check is triggered.

Signed-off-by: Zhang Yanmin <yanmin.zhang@intel.com>
Signed-off-by: Yao Jun      <junx.yao@intel.com>
Signed-off-by: Tony Luck    <tony.luck@intel.com>