kasan: Add strscpy() test to trigger tag fault on arm64

When we invoke strscpy() with a maximum size of N bytes, it assumes
that:
- It can always read N bytes from the source.
- It always write N bytes (zero-padded) to the destination.

On aarch64 with Memory Tagging Extension enabled if we pass an N that is
bigger then the source buffer, it would previously trigger an MTE fault.

Implement a KASAN KUnit test that triggers the issue with the previous
implementation of read_word_at_a_time() on aarch64 with MTE enabled.

Cc: Will Deacon <[email protected]>
Signed-off-by: Vincenzo Frascino <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>
Co-developed-by: Peter Collingbourne <[email protected]>
Signed-off-by: Peter Collingbourne <[email protected]>
Reviewed-by: Andrey Konovalov <[email protected]>
Link: https://linux-review.googlesource.com/id/If88e396b9e7c058c1a4b5a252274120e77b1898a
Reviewed-by: Catalin Marinas <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>

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