kasan: fix build by including kernel.h

The <linux/kasan.h> header relies on _RET_IP_ being defined, and had been
receiving that definition via inclusion of bug.h which includes kernel.h.
However, since f39650de ("kernel.h: split out panic and oops helpers")
that is no longer the case and get the following build error when building
CONFIG_KASAN_HW_TAGS on arm64:

  In file included from arch/arm64/mm/kasan_init.c:10:
  include/linux/kasan.h: In function 'kasan_slab_free':
  include/linux/kasan.h:230:39: error: '_RET_IP_' undeclared (first use in this function)
    230 |   return __kasan_slab_free(s, object, _RET_IP_, init);

Fix it by including kernel.h from kasan.h.

Link: https://lkml.kernel.org/r/20210705072716.2125074-1-elver@google.com
Fixes: f39650de

 ("kernel.h: split out panic and oops helpers")
Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Peter Collingbourne <pcc@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>