Smack: slab-out-of-bounds in vsscanf

commit 84e99e58

 upstream.

Add barrier to soob. Return -EOVERFLOW if the buffer
is exceeded.

Suggested-by: Hillf Danton <hdanton@sina.com>
Reported-by:  <syzbot+bfdd4a2f07be52351350@syzkaller.appspotmail.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>