net: vlan: update wrong comments

vlan_insert_tag() and friends do not allocate a new skb.
However they might allocate a new skb->head.
Update their comments to better describe their behavior.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_vlan.h

@@ -408,7 +408,7 @@ static inline int __vlan_insert_tag(struct sk_buff *skb,
  * @mac_len: MAC header length including outer vlan headers
  *
  * Inserts the VLAN tag into @skb as part of the payload at offset mac_len
- * Returns a VLAN tagged skb. If a new skb is created, @skb is freed.
+ * Returns a VLAN tagged skb. This might change skb->head.
  *
  * Following the skb_unshare() example, in case of error, the calling function
  * doesn't have to worry about freeing the original skb.
@@ -437,7 +437,7 @@ static inline struct sk_buff *vlan_insert_inner_tag(struct sk_buff *skb,
  * @vlan_tci: VLAN TCI to insert
  *
  * Inserts the VLAN tag into @skb as part of the payload
- * Returns a VLAN tagged skb. If a new skb is created, @skb is freed.
+ * Returns a VLAN tagged skb. This might change skb->head.
  *
  * Following the skb_unshare() example, in case of error, the calling function
  * doesn't have to worry about freeing the original skb.
@@ -457,7 +457,7 @@ static inline struct sk_buff *vlan_insert_tag(struct sk_buff *skb,
  * @vlan_tci: VLAN TCI to insert
  *
  * Inserts the VLAN tag into @skb as part of the payload
- * Returns a VLAN tagged skb. If a new skb is created, @skb is freed.
+ * Returns a VLAN tagged skb. This might change skb->head.
  *
  * Following the skb_unshare() example, in case of error, the calling function
  * doesn't have to worry about freeing the original skb.