datapath: fix a memory leak

The user_skb maybe be leaked if the operation on it failed and codes
skipped into the label "out:" without calling genlmsg_unicast.

Cc: Pravin Shelar <pshelar@nicira.com>
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>