loopback: sctp: add NETIF_F_SCTP_CSUM to device features

Drivers are allowed to set NETIF_F_SCTP_CSUM if they have
hardware crc32c checksumming support for the SCTP protocol.
Currently, NETIF_F_SCTP_CSUM flag is available in igb,
ixgbe, i40e/i40evf drivers and for vlan devices.

If we don't have NETIF_F_SCTP_CSUM then crc32c is done
through CPU instructions, invoked from crypto layer, or
if not available as slow-path fallback in software.

Currently, loopback device propagates checksum offloading
feature flags in dev->features, but is missing SCTP checksum
offloading. Therefore, account for NETIF_F_SCTP_CSUM as
well.

Before patch:

./netperf_sctp -H 192.168.0.100 -t SCTP_STREAM_MANY
SCTP 1-TO-MANY STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 192.168.0.100 () port 0 AF_INET
Recv   Send    Send
Socket Socket  Message  Elapsed
Size   Size    Size     Time     Throughput
bytes  bytes   bytes    secs.    10^6bits/sec

4194304 4194304   4096    10.00    4683.50

After patch:

./netperf_sctp -H 192.168.0.100 -t SCTP_STREAM_MANY
SCTP 1-TO-MANY STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 192.168.0.100 () port 0 AF_INET
Recv   Send    Send
Socket Socket  Message  Elapsed
Size   Size    Size     Time     Throughput
bytes  bytes   bytes    secs.    10^6bits/sec

4194304 4194304   4096    10.00    15348.26

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>