net: add pskb_may_pull_reason() helper

pskb_may_pull() can fail for two different reasons.

Provide pskb_may_pull_reason() helper to distinguish
between these reasons.

It returns:

SKB_NOT_DROPPED_YET           : Success
SKB_DROP_REASON_PKT_TOO_SMALL : packet too small
SKB_DROP_REASON_NOMEM         : skb->head could not be resized
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/skbuff.h

@@ -2631,13 +2631,24 @@ void *skb_pull_data(struct sk_buff *skb, size_t len);
 
 void *__pskb_pull_tail(struct sk_buff *skb, int delta);
 
-static inline bool pskb_may_pull(struct sk_buff *skb, unsigned int len)
+static inline enum skb_drop_reason
+pskb_may_pull_reason(struct sk_buff *skb, unsigned int len)
 {
 	if (likely(len <= skb_headlen(skb)))
-		return true;
+		return SKB_NOT_DROPPED_YET;
+
 	if (unlikely(len > skb->len))
-		return false;
-	return __pskb_pull_tail(skb, len - skb_headlen(skb)) != NULL;
+		return SKB_DROP_REASON_PKT_TOO_SMALL;
+
+	if (unlikely(!__pskb_pull_tail(skb, len - skb_headlen(skb))))
+		return SKB_DROP_REASON_NOMEM;
+
+	return SKB_NOT_DROPPED_YET;
+}
+
+static inline bool pskb_may_pull(struct sk_buff *skb, unsigned int len)
+{
+	return pskb_may_pull_reason(skb, len) == SKB_NOT_DROPPED_YET;
 }
 
 static inline void *pskb_pull(struct sk_buff *skb, unsigned int len)