macvlan: Add support for 'always_on' offload features

Macvlan currently inherits all of its features from the lower
device.  When lower device disables offload support, this causes
macvlan to disable offload support as well.  This causes
performance regression when using macvlan/macvtap in bridge
mode.

It can be easily demonstrated by creating 2 namespaces using
macvlan in bridge mode and running netperf between them:

MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 10.0.0.1 () port 0 AF_INET
Recv   Send    Send
Socket Socket  Message  Elapsed
Size   Size    Size     Time     Throughput
bytes  bytes   bytes    secs.    10^6bits/sec

 87380  16384  16384    20.00    1204.61

To restore the performance, we add software offload features
to the list of "always_on" features for macvlan.  This way
when a namespace or a guest using macvtap initially sends a
packet, this packet will not be segmented at macvlan level.
It will only be segmented when macvlan sends the packet
to the lower device.

MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 10.0.0.1 () port 0 AF_INET
Recv   Send    Send
Socket Socket  Message  Elapsed
Size   Size    Size     Time     Throughput
bytes  bytes   bytes    secs.    10^6bits/sec

 87380  16384  16384    20.00    5507.35

Fixes: 6acf54f1 (macvtap: Add support of packet capture on macvtap device.)
Fixes: 797f87f8 (macvlan: fix netdev feature propagation from lower device)
CC: Florian Westphal <fw@strlen.de>
CC: Christian Borntraeger <borntraeger@de.ibm.com>
CC: Jason Wang <jasowang@redhat.com>
CC: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>