proc: fix /proc/*/pagemap some more

struct pagemap_walk was placed on stack, some hooks are initialized, the
rest (->pgd_entry, ->pud_entry, ->pte_entry) are valid but junk.

Reported-by: Eric Sesterhenn <[email protected]>
Signed-off-by: Alexey Dobriyan <[email protected]>
Cc: "Vegard Nossum" <[email protected]>
Cc: <[email protected]> [2.6.25.x, 2.6.26.x]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/proc/task_mmu.c

@@ -636,7 +636,7 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
 	struct pagemapread pm;
 	int pagecount;
 	int ret = -ESRCH;
-	struct mm_walk pagemap_walk;
+	struct mm_walk pagemap_walk = {};
 	unsigned long src;
 	unsigned long svpfn;
 	unsigned long start_vaddr;