sctp: Rename NETIF_F_SCTP_CSUM to NETIF_F_SCTP_CRC

The SCTP checksum is really a CRC and is very different from the
standards 1's complement checksum that serves as the checksum
for IP protocols. This offload interface is also very different.
Rename NETIF_F_SCTP_CSUM to NETIF_F_SCTP_CRC to highlight these
differences. The term CSUM should be reserved in the stack to refer
to the standard 1's complement IP checksum.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>