LinuxKPI: 802.11: deal with scan_ie_len

We only need to reserve the extra space for DSSS if
NL80211_FEATURE_DS_PARAM_SET_IE_IN_PROBES is set, so add the conditional.
Also add checks in case scan_ie_len will grow beyond the maximum.
Given this is currently unlikely, leave the cleanup for later as
some other restructuring should be done first.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: cc
Differential Revision: https://reviews.freebsd.org/D42425