proc: fix /proc/*/pagemap some more

proc: fix /proc/*/pagemap some more

commit ee1e6ab6056a8b9c19377257002da98b83819531 upstream

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 <snakebyte@gmx.de>
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: "Vegard Nossum" <vegard.nossum@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>