do_wp_page: fix regression with execute in place

Fix do_wp_page for VM_MIXEDMAP mappings.

In the case where pfn_valid returns 0 for a pfn at the beginning of
do_wp_page and the mapping is not shared writable, the code branches to
label `gotten:' with old_page == NULL.

In case the vma is locked (vma->vm_flags & VM_LOCKED), lock_page,
clear_page_mlock, and unlock_page try to access the old_page.

This patch checks whether old_page is valid before it is dereferenced.

The regression was introduced by "mlock: mlocked pages are unevictable"
(commit b291f000).
Signed-off-by: Carsten Otte <cotte@de.ibm.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: <stable@kernel.org>		[2.6.28.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -1999,7 +1999,7 @@ gotten:
 	 * Don't let another task, with possibly unlocked vma,
 	 * keep the mlocked page.
 	 */
-	if (vma->vm_flags & VM_LOCKED) {
+	if ((vma->vm_flags & VM_LOCKED) && old_page) {
 		lock_page(old_page);	/* for LRU manipulation */
 		clear_page_mlock(old_page);
 		unlock_page(old_page);