wifi: iwlwifi: Track scan_cmd allocation size explicitly

In preparation for reducing the use of ksize(), explicitly track the
size of scan_cmd allocations. This also allows for noticing if the scan
size changes unexpectedly. Note that using ksize() was already incorrect
here, in the sense that ksize() would not match the actual allocation
size, which would trigger future run-time allocation bounds checking.
(In other words, memset() may know how large scan_cmd was allocated for,
but ksize() will return the upper bounds of the actually allocated memory,
causing a run-time warning about an overflow.)

Cc: Gregory Greenman <gregory.greenman@intel.com>
Cc: Kalle Valo <kvalo@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Luca Coelho <luciano.coelho@intel.com>
Cc: Johannes Berg <johannes.berg@intel.com>
Cc: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Cc: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Cc: Ilan Peer <ilan.peer@intel.com>
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220923220853.3302056-1-keescook@chromium.org