ixgbe: move headroom initialization to ixgbe_configure_rx_ring

ixgbe_rx_offset(), that is supposed to initialize the Rx buffer headroom,
relies on __IXGBE_RX_BUILD_SKB_ENABLED flag.

Currently, the callsite of mentioned function is placed incorrectly
within ixgbe_setup_rx_resources() 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 ixgbe_rx_offset() to ixgbe_configure_rx_ring() after
the flag setting, which happens to be set in ixgbe_set_rx_buffer_len.

Fixes: c0d4e9d2 ("ixgbe: store the result of ixgbe_rx_offset() onto ixgbe_ring")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: Vishakha Jambekar <vishakha.jambekar@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>