net: netsec: Correct dma sync for XDP_TX frames

bpf_xdp_adjust_head() can change the frame boundaries. Account for the
potential shift properly by calculating the new offset before
syncing the buffer to the device for XDP_TX

Fixes: ba2b2321

 ("net: netsec: add XDP support")
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>