lzo: check for length overrun in variable length encoding.

This fix ensures that we never meet an integer overflow while adding
255 while parsing a variable length encoding. It works differently from
commit 206a81c ("lzo: properly check for overruns") because instead of
ensuring that we don't overrun the input, which is tricky to guarantee
due to many assumptions in the code, it simply checks that the cumulated
number of 255 read cannot overflow by bounding this number.

The MAX_255_COUNT is the maximum number of times we can add 255 to a base
count without overflowing an integer. The multiply will overflow when
multiplying 255 by more than MAXINT/255. The sum will overflow earlier
depending on the base count. Since the base count is taken from a u8
and a few bits, it is safe to assume that it will always be lower than
or equal to 2*255, thus we can always prevent any overflow by accepting
two less 255 steps.

This patch also reduces the CPU overhead and actually increases performance
by 1.1% compared to the initial code, while the previous fix costs 3.1%
(measured on x86_64).

The fix needs to be backported to all currently supported stable kernels.

Reported-by: Willem Pinckaers <willem@lekkertech.net>
Cc: "Don A. Bailey" <donb@securitymouse.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
Documentation: lzo: document part of the encoding

Add a complete description of the LZO format as processed by the
decompressor. I have not found a public specification of this format
hence this analysis, which will be used to better understand the code.

Cc: Willem Pinckaers <willem@lekkertech.net>
Cc: "Don A. Bailey" <donb@securitymouse.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>