Wrong page freed on preallocate_pmds() failure exit

Note that pmds[i] is simply uninitialized at that point...

Granted, it's very hard to hit (you need split page locks *and*
kmalloc(sizeof(spinlock_t), GFP_KERNEL) failing), but the code is
obviously bogus.

Introduced by commit 09ef4939 ("x86: add missed
pgtable_pmd_page_ctor/dtor calls for preallocated pmds")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/mm/pgtable.c

@@ -209,7 +209,7 @@ static int preallocate_pmds(pmd_t *pmds[])
 		if (!pmd)
 			failed = true;
 		if (pmd && !pgtable_pmd_page_ctor(virt_to_page(pmd))) {
-			free_page((unsigned long)pmds[i]);
+			free_page((unsigned long)pmd);
 			pmd = NULL;
 			failed = true;
 		}