wifi: mac80211: Set n_channels after allocating struct cfg80211_scan_request

In the Linux kernel, the following vulnerability has been resolved: wifi: mac80211: Set n_channels after allocating struct cfg80211_scan_request Make sure that n_channels is set after allocating the struct cfg80211_registered_device::int_scan_req member. Seen with syzkaller: UBSAN: array-index-out-of-bounds in net/mac80211/scan.c:1208:5 index 0 is out of range for type 'struct ieee80211_channel *[] __counted_by(n_channels)' (aka 'struct ieee80211_channel *[]') This was missed in the initial conversions because I failed to locate the allocation likely due to the "sizeof(void *)" not matching the "channels" array type.