x86, mm: Merge alloc_low_page between 64bit and 32bit

They are almost same except 64 bit need to handle after_bootmem case.

Add mm_internal.h to make that alloc_low_page() only to be accessible
from arch/x86/mm/init*.c

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1353123563-3103-25-git-send-email-yinghai@kernel.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/mm/mm_internal.h

@@ -0,0 +1,6 @@
+#ifndef __X86_MM_INTERNAL_H
+#define __X86_MM_INTERNAL_H
+
+void *alloc_low_page(void);
+
+#endif	/* __X86_MM_INTERNAL_H */