net: make net_get_random_once irq safe

I initial build non irq safe version of net_get_random_once because I
would liked to have the freedom to defer even the extraction process of
get_random_bytes until the nonblocking pool is fully seeded.

I don't think this is a good idea anymore and thus this patch makes
net_get_random_once irq safe. Now someone using net_get_random_once does
not need to care from where it is called.

Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/net.h

@@ -250,7 +250,6 @@ bool __net_get_random_once(void *buf, int nbytes, bool *done,
 #define ___NET_RANDOM_STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 #endif /* HAVE_JUMP_LABEL */
 
-/* BE CAREFUL: this function is not interrupt safe */
 #define net_get_random_once(buf, nbytes)				\
 	({								\
 		bool ___ret = false;					\

net/core/utils.c

@@ -370,16 +370,17 @@ bool __net_get_random_once(void *buf, int nbytes, bool *done,
 			   struct static_key *done_key)
 {
 	static DEFINE_SPINLOCK(lock);
+	unsigned long flags;
 
-	spin_lock_bh(&lock);
+	spin_lock_irqsave(&lock, flags);
 	if (*done) {
-		spin_unlock_bh(&lock);
+		spin_unlock_irqrestore(&lock, flags);
 		return false;
 	}
 
 	get_random_bytes(buf, nbytes);
 	*done = true;
-	spin_unlock_bh(&lock);
+	spin_unlock_irqrestore(&lock, flags);
 
 	__net_random_once_disable_jump(done_key);