igc: fix missing update of skb->tail in igc_xmit_frame()

igc_xmit_frame() misses updating skb->tail when the packet size is
shorter than the minimum one.
Use skb_put_padto() in alignment with other Intel Ethernet drivers.

Fixes: 0507ef8a0372 ("igc: Add transmit and receive fastpath and interrupt handlers")
Signed-off-by: Kohei Enju <kohei@enjuk.jp>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>