net: don't clear IFF_XMIT_DST_RELEASE in ether_setup

d8873315 introduced the IFF_SKB_TX_SHARING flag, which I unilaterally set in
ether_setup.  In doing this I didn't realize that other flags (such as
IFF_XMIT_DST_RELEASE) might be set prior to calling the ether_setup routine.
This patch changes ether_setup to or in SKB_TX_SHARING so as not to
inadvertently clear other existing flags.  Thanks to Pekka Riikonen for pointing
out my error

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Reported-by: Pekka Riikonen <priikone@iki.fi>
CC: "David S. Miller" <davem@davemloft.net>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>