wifi: nl80211: fix bounds checker error in nl80211_parse_sched_scan

wifi: nl80211: fix bounds checker error in nl80211_parse_sched_scan

The channels array in the cfg80211_scan_request has a __counted_by
attribute attached to it, which points to the n_channels variable. This
attribute is used in bounds checking, and if it is not set before the
array is filled, then the bounds sanitizer will issue a warning or a
kernel panic if CONFIG_UBSAN_TRAP is set.

This patch sets the size of allocated memory as the initial value for
n_channels. It is updated with the actual number of added elements after
the array is filled.

Fixes: aa4ec06c455d ("wifi: cfg80211: use __counted_by where appropriate")
Cc: stable@vger.kernel.org
Signed-off-by: Aleksei Vetrov <vvvvvv@google.com>
Reviewed-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Link: https://patch.msgid.link/20241029-nl80211_parse_sched_scan-bounds-checker-fix-v2-1-c804b787341f@google.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>