mac80211: make align adjustment code support paged SKB

This fixed a BUG_ON in __skb_trim() when paged rx is used in
iwlwifi driver. Yes, the whole mac80211 stack doesn't support
paged SKB yet. But let's start the work slowly from small
code snippets.

Reported-and-tested-by: Abhijeet Kolekar <abhijeet.kolekar@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>