In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memcpy(), memmove(), and memset(), avoid
intentionally writing across neighboring array fields.

Validate the expected key size and introduce a wrapping structure
to use as the multi-field memcpy() destination so that overflows
can be correctly detected.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210616203952.1248910-1-keescook@chromium.org