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.