crc_folding: Fix potential out-of-bounds access

crc_folding: Fix potential out-of-bounds access

In some (very rare) scenarios, the SIMD code in the `crc_folding` module
can perform out-of-bounds reads or writes, which could lead to GPF
crashes.

Here's the deal: when the `crc_fold_copy` function is called with a
non-zero `len` argument of less then 16, `src` is read through
`_mm_loadu_si128` which always reads 16 bytes. If the `src` pointer
points to a location which contains `len` bytes, but any of the `16 -
len` out-of-bounds bytes falls in unmapped memory, this operation will
trigger a GPF.

The same goes for the `dst` pointer when written to through
`_mm_storeu_si128`.

With this patch applied, the crash no longer occurs.

We first discovered this issue though Valgrind reporting an
out-of-bounds access while running a unit-test for some code derived
from `crc_fold_copy`. In general, the out-of-bounds read is not an issue
because reads only occur in sections which are definitely mapped
(assuming page size is a multiple of 16), and garbage bytes are ignored.
While giving this some more thought we realized for small `len` values
and `src` or `dst` pointers at a very specific place in the address
space can lead to GPFs.

- Minor tweaks to merge request by Jim Kukunas <james.t.kukunas@linux.intel.com>
	- removed C11-isms
	- use unaligned load
	- better integrated w/ zlib (use zalign)
	- removed full example code from commit msg