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>