mm/CMA: fix boot regression due to physical address of high_memory

high_memory isn't direct mapped memory so retrieving it's physical address
isn't appropriate.  But, it would be useful to check physical address of
highmem boundary so it's justfiable to get physical address from it.  In
x86, there is a validation check if CONFIG_DEBUG_VIRTUAL and it triggers
following boot failure reported by Ingo.

  ...
  BUG: Int 6: CR2 00f06f53
  ...
  Call Trace:
    dump_stack+0x41/0x52
    early_idt_handler+0x6b/0x6b
    cma_declare_contiguous+0x33/0x212
    dma_contiguous_reserve_area+0x31/0x4e
    dma_contiguous_reserve+0x11d/0x125
    setup_arch+0x7b5/0xb63
    start_kernel+0xb8/0x3e6
    i386_start_kernel+0x79/0x7d

To fix boot regression, this patch implements workaround to avoid
validation check in x86 when retrieving physical address of high_memory.
__pa_nodebug() used by this patch is implemented only in x86 so there is
no choice but to use dirty #ifdef.

[akpm@linux-foundation.org: tweak comment]
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Reported-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Ingo Molnar <mingo@kernel.org>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>