net: document skb parameter in function 'skb_gso_size_check'

Remove kernel-doc warning:

  net/core/skbuff.c:4953: warning: Function parameter or member 'skb' not described in 'skb_gso_size_check'

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: David S. Miller <davem@davemloft.net>