sk_buff.h: improve description of CHECKSUM_{COMPLETE, UNNECESSARY}

Add FCoE to the list of protocols that can set CHECKSUM_UNNECESSARY; add a
note to CHECKSUM_COMPLETE section to specify that it does not apply to SCTP
and FCoE protocols.

Suggested-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -109,6 +109,7 @@
  *       may perform further validation in this case.
  *     GRE: only if the checksum is present in the header.
  *     SCTP: indicates the CRC in SCTP header has been validated.
+ *     FCOE: indicates the CRC in FC frame has been validated.
  *
  *   skb->csum_level indicates the number of consecutive checksums found in
  *   the packet minus one that have been verified as CHECKSUM_UNNECESSARY.
@@ -126,8 +127,10 @@
  *   packet as seen by netif_rx() and fills out in skb->csum. Meaning, the
  *   hardware doesn't need to parse L3/L4 headers to implement this.
  *
- *   Note: Even if device supports only some protocols, but is able to produce
- *   skb->csum, it MUST use CHECKSUM_COMPLETE, not CHECKSUM_UNNECESSARY.
+ *   Notes:
+ *   - Even if device supports only some protocols, but is able to produce
+ *     skb->csum, it MUST use CHECKSUM_COMPLETE, not CHECKSUM_UNNECESSARY.
+ *   - CHECKSUM_COMPLETE is not applicable to SCTP and FCoE protocols.
  *
  * CHECKSUM_PARTIAL:
  *