IB/core: Pass imm_data from ib_uverbs_send_wr to ib_send_wr correctly

Currently, we don't copy the immediate data from the userspace struct
to the kernel one when UD messages are being sent.

This patch makes sure that the immediate data is set correctly.

Signed-off-by: Latchesar Ionkov <lucho@ionkov.net>
Signed-off-by: Roland Dreier <roland@purestorage.com>