x86, mm: Make sure to find a 2M free block for the first mapped area

Henrik reported that his MacAir 3.1 would not boot with

| commit 8d57470d
| Date:   Fri Nov 16 19:38:58 2012 -0800
|
|    x86, mm: setup page table in top-down

It turns out that we do not calculate the real_end properly:
We try to get 2M size with 4K alignment, and later will round down
to 2M, so we will get less then 2M for first mapping, in extreme
case could be only 4K only. In Henrik's system it has (1M-32K) as
last usable rage is [mem 0x7f9db000-0x7fef8fff].

The problem is exposed when EFI booting have several holes and it
will force mapping to use PTE instead as we only map usable areas.

To fix it, just make it be 2M aligned, so we can be guaranteed to be
able to use large pages to map it.
Reported-by: NHenrik Rydberg <rydberg@euromail.se>
Bisected-by: NHenrik Rydberg <rydberg@euromail.se>
Tested-by: NHenrik Rydberg <rydberg@euromail.se>
Signed-off-by: NYinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/CAE9FiQX4nQ7_1kg5RL_vh56rmcSHXUi1ExrZX7CwED4NGMnHfg@mail.gmail.comSigned-off-by: NH. Peter Anvin <hpa@zytor.com>

arch/x86/mm/init.c

@@ -410,9 +410,8 @@ void __init init_mem_mapping(void)
 	/* the ISA range is always mapped regardless of memory holes */
 	init_memory_mapping(0, ISA_END_ADDRESS);
 
-	/* xen has big range in reserved near end of ram, skip it at first */
-	addr = memblock_find_in_range(ISA_END_ADDRESS, end, PMD_SIZE,
-			 PAGE_SIZE);
+	/* xen has big range in reserved near end of ram, skip it at first.*/
+	addr = memblock_find_in_range(ISA_END_ADDRESS, end, PMD_SIZE, PMD_SIZE);
 	real_end = addr + PMD_SIZE;
 
 	/* step_size need to be small so pgt_buf from BRK could cover it */