genetlink: Add lockdep_genl_is_held().

Open vSwitch uses genl_mutex locking to protect datapath
data-structures like flow-table, flow-actions. Following patch adds
lockdep_genl_is_held() which is used for rcu annotation to prove
locking.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>