genetlink: define lockdep_genl_is_held() when CONFIG_LOCKDEP

lockdep_is_held() is defined when CONFIG_LOCKDEP, not CONFIG_PROVE_LOCKING.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jesse Gross <jesse@nicira.com>
Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>