include: net: add static inline dst_dev_overhead() to dst.h

Add static inline dst_dev_overhead() function to include/net/dst.h. This
helper function is used by ioam6_iptunnel, rpl_iptunnel and
seg6_iptunnel to get the dev's overhead based on a cache entry
(dst_entry). If the cache is empty, the default and generic value
skb->mac_len is returned. Otherwise, LL_RESERVED_SPACE() over dst's dev
is returned.

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Cc: Alexander Lobakin <aleksander.lobakin@intel.com>
Cc: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/net/dst.h

@@ -440,6 +440,15 @@ static inline void dst_set_expires(struct dst_entry *dst, int timeout)
 		dst->expires = expires;
 }
 
+static inline unsigned int dst_dev_overhead(struct dst_entry *dst,
+					    struct sk_buff *skb)
+{
+	if (likely(dst))
+		return LL_RESERVED_SPACE(dst->dev);
+
+	return skb->mac_len;
+}
+
 INDIRECT_CALLABLE_DECLARE(int ip6_output(struct net *, struct sock *,
 					 struct sk_buff *));
 INDIRECT_CALLABLE_DECLARE(int ip_output(struct net *, struct sock *,