kasan: initialize shadow to 0xff for tag-based mode

kasan: initialize shadow to 0xff for tag-based mode

A tag-based KASAN shadow memory cell contains a memory tag, that
corresponds to the tag in the top byte of the pointer, that points to that
memory.  The native top byte value of kernel pointers is 0xff, so with
tag-based KASAN we need to initialize shadow memory to 0xff.

[[email protected]: arm64: skip kmemleak for KASAN again\
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/5cc1b789aad7c99cf4f3ec5b328b147ad53edb40.1544099024.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <[email protected]>
Reviewed-by: Andrey Ryabinin <[email protected]>
Reviewed-by: Dmitry Vyukov <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>