netdev: Safely increment refcount in netdev_open().

netdev_open() would previously increment a netdev's refcount without
holding a lock for it. This commit shifts the locking to protect it.

Found by inspection.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>