CHROMIUM: fix stack memory contents leak in UNAME26

CHROMIUM: fix stack memory contents leak in UNAME26

The UNAME26 personality allows a leak of kernel stack contents. This
fixes it by initializing the stack buffer to zero and truncating the
copy_to_user correctly.

BUG=None
TEST=parrot build, verified proper wiping of stack contents under UNAME26

Change-Id: Iab2ff3b111596db897b33e45de9c5d0b5db9849c
[submitting upstream]
Reported-by: Brad Spengler <spender@grsecurity.net>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/34482
Reviewed-by: Olof Johansson <olofj@chromium.org>