IB/core: Support rate limit for packet pacing

Add new member rate_limit to ib_qp_attr which holds the packet pacing rate
in kbps, 0 means unlimited.

IB_QP_RATE_LIMIT is added to ib_attr_mask and could be used by RAW
QPs when changing QP state from RTR to RTS, RTS to RTS.

Signed-off-by: Bodong Wang <bodong@mellanox.com>
Reviewed-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>