kasan: fix unit tests with CONFIG_UBSAN_LOCAL_BOUNDS enabled

These tests deliberately access these arrays out of bounds, which will
cause the dynamic local bounds checks inserted by
CONFIG_UBSAN_LOCAL_BOUNDS to fail and panic the kernel.  To avoid this
problem, access the arrays via volatile pointers, which will prevent the
compiler from being able to determine the array bounds.

These accesses use volatile pointers to char (char *volatile) rather than
the more conventional pointers to volatile char (volatile char *) because
we want to prevent the compiler from making inferences about the pointer
itself (i.e.  its array bounds), not the data that it refers to.

Link: https://lkml.kernel.org/r/20210507025915.1464056-1-pcc@google.com
Link: https://linux-review.googlesource.com/id/I90b1713fbfa1bf68ff895aef099ea77b98a7c3b9


Signed-off-by: Peter Collingbourne <pcc@google.com>
Tested-by: Alexander Potapenko <glider@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Peter Collingbourne <pcc@google.com>
Cc: George Popescu <georgepope@android.com>
Cc: Elena Petrova <lenaptr@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>