Change snprintf to scnprintf. There are generally two cases where using
snprintf causes problems.

1) Uses of size += snprintf(buf, SIZE - size, fmt, ...)  In this case,
if snprintf would have written more characters than what the buffer
size (SIZE) is, then size will end up larger than SIZE. In later uses
of snprintf, SIZE - size will result in a negative number, leading to
problems. Note that size might already be too large by using size =
snprintf before the code reaches a case of size += snprintf.

2) If size is ultimately used as a length parameter for a copy back to
user space, then it will potentially allow for a buffer overflow and
information disclosure when size is greater than SIZE. When the size is
used to index the buffer directly, we can have memory corruption. This
also means when size = snprintf... is used, it may also cause problems
since size may become large. Copying to userspace is mitigated by the
HARDENED_USERCOPY kernel configuration.

The solution to these issues is to use scnprintf which returns the number
of characters actually written to the buffer, so the size variable will
never exceed SIZE.

Cc: Willy Tarreau <w@1wt.eu>
Cc: Silvio Cesare <silvio.cesare@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>