RDMA/rxe: Improve loopback marking

Currently a packet is marked for loopback only if the source and
destination addresses equals. This is not enough when multiple gids are
present in rxe device's gid table and the traffic is from one gid to
another. Fix it by marking the packet for loopback if the destination MAC
address is equal to the source MAC address.

Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Tested-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>