wifi: mac80211: update scratch_pos after defrag

wifi: mac80211: update scratch_pos after defrag

The scratch_pos update here was lost after defrag, so any
other uses of the scratch buffer might overwrite it.

Fixes: a286de1aa38f ("wifi: mac80211: Rename multi_link")
Reviewed-by: Benjamin Berg <benjamin.berg@intel.com>
Reviewed-by: Ilan Peer <ilan.peer@intel.com>
Reviewed-by: Miriam Rachel Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20240228094901.9da35f39eeb7.I7127f2918ec4cba416fcbc35eacaea10262c1268@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>