IB/core: Initialize UD header structure with IP and UDP headers

ib_ud_header_init() is used to format InfiniBand headers
in a buffer up to (but not with) BTH. For RoCE UDP ENCAP it is
required that this function would be able to build also IP and UDP
headers.

Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>