kasan: clean up kasan_requires_meta

kasan: clean up kasan_requires_meta

Currently, for Generic KASAN mode, kasan_requires_meta is defined to
return kasan_stack_collection_enabled.

Even though the Generic mode does not support disabling stack trace
collection, kasan_requires_meta was implemented in this way to make it
easier to implement the disabling for the Generic mode in the future.

However, for the Generic mode, the per-object metadata also stores the
quarantine link.  So even if disabling stack collection is implemented,
the per-object metadata will still be required.

Fix kasan_requires_meta to return true for the Generic mode and update the
related comments.

This change does not fix any observable bugs but rather just brings the
code to a cleaner state.

Link: https://lkml.kernel.org/r/8086623407095ac1c82377a2107dcc5845f99cfa.1703188911.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>