hugetlb: add missing unlock in avoidcopy path in hugetlb_cow()

This patch fixes possible deadlock in hugepage lock_page()
by adding missing unlock_page().

libhugetlbfs test will hit this bug when the next patch in this
patchset ("hugetlb, HWPOISON: move PG_HWPoison bit check") is applied.
Signed-off-by: NNaoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: NJun'ichi Nomura <j-nomura@ce.jp.nec.com>
Acked-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NAndi Kleen <ak@linux.intel.com>

mm/hugetlb.c

@@ -2324,9 +2324,11 @@ static int hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma,
 	 * and just make the page writable */
 	avoidcopy = (page_mapcount(old_page) == 1);
 	if (avoidcopy) {
-		if (!trylock_page(old_page))
+		if (!trylock_page(old_page)) {
 			if (PageAnon(old_page))
 				page_move_anon_rmap(old_page, vma, address);
+		} else
+			unlock_page(old_page);
 		set_huge_ptep_writable(vma, address, ptep);
 		return 0;
 	}