net: use skb_is_gso_sctp() instead of open-coding

As well as the basic conversion, I noticed that a lot of the
SCTP code checks gso_type without first checking skb_is_gso()
so I have added that where appropriate.

Also, document the helper.

Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/Documentation/networking/segmentation-offloads.txt b/Documentation/networking/segmentation-offloads.txt
index fc0c949..aca542e 100644
--- a/Documentation/networking/segmentation-offloads.txt
+++ b/Documentation/networking/segmentation-offloads.txt
@@ -155,7 +155,10 @@
 possibility that gso_size will be GSO_BY_FRAGS and handle that case
 appropriately.
 
-There are a couple of helpers to make this easier:
+There are some helpers to make this easier:
+
+ - skb_is_gso(skb) && skb_is_gso_sctp(skb) is the best way to see if
+   an skb is an SCTP GSO skb.
 
  - For size checks, the skb_gso_validate_*_len family of helpers correctly
    considers GSO_BY_FRAGS.