mm: fix potential infinite loop in dissolve_free_huge_pages()

commit d0177639 upstream.

It is possible for some platforms, such as powerpc to set HPAGE_SHIFT to
0 to indicate huge pages not supported.

When this is the case, hugetlbfs could be disabled during boot time:
hugetlbfs: disabling because there are no supported hugepage sizes

Then in dissolve_free_huge_pages(), order is kept maximum (64 for
64bits), and the for loop below won't end: for (pfn = start_pfn; pfn <
end_pfn; pfn += 1 << order)

As suggested by Naoya, below fix checks hugepages_supported() before
calling dissolve_free_huge_pages().

[rientjes@google.com: no legitimate reason to call dissolve_free_huge_pages() when !hugepages_supported()]
Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
Acked-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[ kamal: 3.13-stable prereq for
  641844f5 mm/hugetlb: introduce minimum hugepage order ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

mm/hugetlb.c

@@ -920,6 +920,9 @@ void dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
 	unsigned long pfn;
 	struct hstate *h;
 
+	if (!hugepages_supported())
+		return;
+
 	/* Set scan step to minimum hugepage size */
 	for_each_hstate(h)
 		if (order > huge_page_order(h))