veth: Update XDP feature set when bringing up device

There's an early return in veth_set_features() if the device is in a down
state, which leads to the XDP feature flags not being updated when enabling
GRO while the device is down. Which in turn leads to XDP_REDIRECT not
working, because the redirect code now checks the flags.

Fix this by updating the feature flags after bringing the device up.

Before this patch:

NETDEV_XDP_ACT_BASIC:		yes
NETDEV_XDP_ACT_REDIRECT:	yes
NETDEV_XDP_ACT_NDO_XMIT:	no
NETDEV_XDP_ACT_XSK_ZEROCOPY:	no
NETDEV_XDP_ACT_HW_OFFLOAD:	no
NETDEV_XDP_ACT_RX_SG:		yes
NETDEV_XDP_ACT_NDO_XMIT_SG:	no

After this patch:

NETDEV_XDP_ACT_BASIC:		yes
NETDEV_XDP_ACT_REDIRECT:	yes
NETDEV_XDP_ACT_NDO_XMIT:	yes
NETDEV_XDP_ACT_XSK_ZEROCOPY:	no
NETDEV_XDP_ACT_HW_OFFLOAD:	no
NETDEV_XDP_ACT_RX_SG:		yes
NETDEV_XDP_ACT_NDO_XMIT_SG:	yes

Fixes: fccca038 ("veth: take into account device reconfiguration for xdp_features flag")
Fixes: 66c0e13a

 ("drivers: net: turn on XDP features")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20230911135826.722295-1-toke@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>