enic: fix offset for Rx mbuf data

enic: fix offset for Rx mbuf data

The code to provide mbufs for RX used m->data_off instead of
RTE_PKTMBUF_HEADROOM as the position inside the mbuf for the data to be
written. As the mbuf is uninitialised, this could potentially cause Rx
data to be placed at the wrong address in the mbuf - or even outside it.

Fixes: 947d860c821f ("enic: improve Rx performance")

Signed-off-by: John Daley <johndale@cisco.com>