mac80211: don't resize skbs needlessly

Header-less cloned skbs with sufficient headroom need not be cloned
unless the tailroom is going to be modified.

Fix ieee80211_skb_resize so it would only resize cloned skbs if either
the header isn't released or the tailroom is going to be modified.

Some drivers might have assumed that skbs are never cloned, so add a HW
flag that explicitly permits cloned TX skbs. Drivers which do not modify
TX skbs should set this flag to avoid copying skbs.

Signed-off-by: Ido Yariv <idox.yariv@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>