genl: Allow concurrent genl callbacks.

All genl callbacks are serialized by genl-mutex. This can become
bottleneck in multi threaded case.
Following patch adds an parameter to genl_family so that a
particular family can get concurrent netlink callback without
genl_lock held.
New rw-sem is used to protect genl callback from genl family unregister.
in case of parallel_ops genl-family read-lock is taken for callbacks and
write lock is taken for register or unregistration for any family.
In case of locked genl family semaphore and gel-mutex is locked for
any openration.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/genetlink.h

@@ -50,6 +50,7 @@ struct genl_family {
 	unsigned int		version;
 	unsigned int		maxattr;
 	bool			netnsok;
+	bool			parallel_ops;
 	int			(*pre_doit)(struct genl_ops *ops,
 					    struct sk_buff *skb,
 					    struct genl_info *info);