After this commit, INTEL_DEBUG=mem will make iris over-allocate a page
for every GEM buffer requested, write a pattern to the unused end area
- which we refer to as the red zone - and check for integrity of the
pattern when the buffer is not used anymore. Once a problem is found,
we dump the buffer and assert(false). Notice that the red zone is not
only the extra page we over-allocate, but it also includes the padding
caused by iris' allocation buckets and page rounding: even if our tool
didn't over-allocate anything (i.e., if we define
RED_ZONE_BUFFER_EXTENSION to zero), the tool would still be doing
helpful work.

While I was able to find an issue in the Kernel with this patch
series, it turns our the Mesa problems I suspected to have found were
issues with this patch. So as of v5 the memory checker still has yet
to find a Mesa memory problem.

I also plan to extend the subsystem to also check for use-after-free
issues in later patches, but first I want to work on the bugs that red
zoning was already able to catch.

v2:
 - Rebase
v3:
 - Missing commas in error message (Marcin Ślusarz).
 - Rely on memcmp() in check_red_zone() instead of implementing our
   own uint64_t check (Marcin Ślusarz).
v4:
 - Rebase.
 - Tweak the dump_mem_range() output after having used it for a while.
 - Make the code a little easier to read.
v5:
 - Use RAW_MODE so we don't get false positives in RBC and HIZ
   auxiliary buffers.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>