IB/IPoIB: Do not set skb truesize since using one linearskb

We are seeing this warning: at net/core/skbuff.c:4174
and before commit a44878d1 ("IB/ipoib: Use one linear skb in RX flow")
skb truesize was not being set when ipoib was using just one skb.
Removing this line avoids the warning when running tcp tests like iperf.

Fixes: a44878d1 ("IB/ipoib: Use one linear skb in RX flow")
Signed-off-by: Carol L Soto <clsoto@linux.vnet.ibm.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>