IB/core: Fix and clean up ib_ud_header_init()

ib_ud_header_init() first clears header and then fills up the various
fields.  Later on, it tests header->immediate_present, which it has
already cleared, so the condition is always false.  Fix this by adding
an immediate_present parameter and setting header->immediate_present
as is done with grh_present.  Also remove unused calculation of
header_len.

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