mac80211_hwsim: allow testing paged RX

Paged RX, i.e. SKBs with (some of) the data in pages instead
of the SKB header data (skb->data) can behave differently in
the stack and cause other bugs. To make debugging easier add
an option to hwsim to test with such SKBs.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>