mm/sparse.c: wrong allocation for mem_section

In commit 83e3c48729d9 ("mm/sparsemem: Allocate mem_section at runtime
for CONFIG_SPARSEMEM_EXTREME=y") mem_section is allocated at runtime to
save memory.

It allocates the first dimension of array with sizeof(struct mem_section).

It costs extra memory, should be sizeof(struct mem_section *).

Fix it.

Link: http://lkml.kernel.org/r/1513932498-20350-1-git-send-email-bhe@redhat.com
Fixes: 83e3c48729 ("mm/sparsemem: Allocate mem_section at runtime for CONFIG_SPARSEMEM_EXTREME=y")
Signed-off-by: Baoquan He <bhe@redhat.com>
Tested-by: Dave Young <dyoung@redhat.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Atsushi Kumagai <ats-kumagai@wm.jp.nec.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/sparse.c

@@ -211,7 +211,7 @@ void __init memory_present(int nid, unsigned long start, unsigned long end)
 	if (unlikely(!mem_section)) {
 		unsigned long size, align;
 
-		size = sizeof(struct mem_section) * NR_SECTION_ROOTS;
+		size = sizeof(struct mem_section*) * NR_SECTION_ROOTS;
 		align = 1 << (INTERNODE_CACHE_SHIFT);
 		mem_section = memblock_virt_alloc(size, align);
 	}