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>