mm/kfence: add a new kunit test test_use_after_free_read_nofault()

mm/kfence: add a new kunit test test_use_after_free_read_nofault()

Faults from copy_from_kernel_nofault() need to be handled by fixup table
and should not be handled by kfence.  Otherwise while reading /proc/kcore
which uses copy_from_kernel_nofault(), kfence can generate false
negatives.  This can happen when /proc/kcore ends up reading an unmapped
address from kfence pool.

Let's add a testcase to cover this case.

Link: https://lkml.kernel.org/r/210e561f7845697a32de44b643393890f180069f.1729272697.git.ritesh.list@gmail.com
Signed-off-by: Nirjhar Roy <nirjhar@linux.ibm.com>
Co-developed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Tested-by: Marco Elver <elver@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>