ip6_gre: fix device features for ioctl setup

When ip6gre is created using ioctl, its features, such as
scatter-gather, GSO and tx-checksumming will be turned off:

  # ip -f inet6 tunnel add gre6 mode ip6gre remote fd00::1
  # ethtool -k gre6 (truncated output)
    tx-checksumming: off
    scatter-gather: off
    tcp-segmentation-offload: off
    generic-segmentation-offload: off [requested on]

But when netlink is used, they will be enabled:
  # ip link add gre6 type ip6gre remote fd00::1
  # ethtool -k gre6 (truncated output)
    tx-checksumming: on
    scatter-gather: on
    tcp-segmentation-offload: on
    generic-segmentation-offload: on

This results in a loss of performance when gre6 is created via ioctl.
The issue was found with LTP/gre tests.

Fix it by moving the setup of device features to a separate function
and invoke it with ndo_init callback because both netlink and ioctl
will eventually call it via register_netdevice():

   register_netdevice()
       - ndo_init() callback -> ip6gre_tunnel_init() or ip6gre_tap_init()
           - ip6gre_tunnel_init_common()
                - ip6gre_tnl_init_features()

The moved code also contains two minor style fixes:
  * removed needless tab from GRE6_FEATURES on NETIF_F_HIGHDMA line.
  * fixed the issue reported by checkpatch: "Unnecessary parentheses around
    'nt->encap.type == TUNNEL_ENCAP_NONE'"

Fixes: ac4eb009

 ("ip6gre: Add support for basic offloads offloads excluding GSO")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>