[IA64] make full use of macro efi_md_size

Macro efi_md_size is defined in efi.c, and here we apply it throughout
efi.c.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>