IPoIB: Add high DMA feature flag

All current InfiniBand devices can handle all DMA addresses, and it's
hard to imagine anyone would be silly enough to build a new device
that couldn't.  Therefore, enable the NETIF_F_HIGHDMA feature for IPoIB.

This has no effect for no, but is needed when we enable gather/scatter
support and checksum stateless offloads.

Signed-off-by: Eli Cohen <eli@mellnaox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>