kasan: only define kasan_metadata_size for Generic mode

kasan: only define kasan_metadata_size for Generic mode

KASAN provides a helper for calculating the size of per-object metadata
stored in the redzone.

As now only the Generic mode uses per-object metadata, only define
kasan_metadata_size() for this mode.

Link: https://lkml.kernel.org/r/8f81d4938b80446bc72538a08217009f328a3e23.1662411799.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <[email protected]>
Reviewed-by: Marco Elver <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Evgenii Stepanov <[email protected]>
Cc: Peter Collingbourne <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>