wifi: nl80211: convert cfg80211_scan_request allocation to *_size macros

wifi: nl80211: convert cfg80211_scan_request allocation to *_size macros

The cfg80211_scan_request structure is followed by a flexible array member
as well as several other arrays that are then stored into pointers in the
structure. These are allocated currently using a simple sequence of
multiplications.

Replace the calculations with struct_size(), size_add(), and array_size()
helper macros. These macros saturate the calculation at SIZE_MAX rather
than overflowing.

Note that we can't use flex_array_size() instead of array_size() because
the fields are not arrays, but simple pointers.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20230228162827.3876606-3-jacob.e.keller@intel.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>