ice: move headroom initialization to ice_setup_rx_ctx

ice_rx_offset(), that is supposed to initialize the Rx buffer headroom,
relies on ICE_RX_FLAGS_RING_BUILD_SKB flag as well as XDP prog presence.

Currently, the callsite of mentioned function is placed incorrectly
within ice_setup_rx_ring() where Rx ring's build skb flag is not
set yet. This causes the XDP_REDIRECT to be partially broken due to
inability to create xdp_frame in the headroom space, as the headroom is
0.

Fix this by moving ice_rx_offset() to ice_setup_rx_ctx() after the flag
setting.

Fixes: f1b1f409 ("ice: store the result of ice_rx_offset() onto ice_ring")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: Kiran Bhandare <kiranx.bhandare@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>