mm/page_owner: avoid null pointer dereference

We have dereferenced page_ext before checking it.  Lets check it first
and then used it.

Fixes: f86e427197 ("mm: check the return value of lookup_page_ext for all call sites")
Link: http://lkml.kernel.org/r/1465249059-7883-1-git-send-email-sudipm.mukherjee@gmail.com
Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_owner.c

@@ -207,13 +207,15 @@ void __dump_page_owner(struct page *page)
 		.nr_entries = page_ext->nr_entries,
 		.entries = &page_ext->trace_entries[0],
 	};
-	gfp_t gfp_mask = page_ext->gfp_mask;
-	int mt = gfpflags_to_migratetype(gfp_mask);
+	gfp_t gfp_mask;
+	int mt;
 
 	if (unlikely(!page_ext)) {
 		pr_alert("There is not page extension available.\n");
 		return;
 	}
+	gfp_mask = page_ext->gfp_mask;
+	mt = gfpflags_to_migratetype(gfp_mask);
 
 	if (!test_bit(PAGE_EXT_OWNER, &page_ext->flags)) {
 		pr_alert("page_owner info is not active (free page?)\n");