crypto: zstd - Annotate struct zstd_ctx with __counted_by

crypto: zstd - Annotate struct zstd_ctx with __counted_by

Add the __counted_by() compiler attribute to the flexible array member
'wksp' to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Use struct_size(), which provides additional compile-time checks for
structures with flexible array members (e.g., __must_be_array()), for
the allocation size for a new 'zstd_ctx' while we're at it.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>