ibcore: 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.

Linux commit:
528e5a1bd3f0e9b760cb3a1062fce7513712a15d

MFC after: 1 week
Reviewed by: kib
Sponsored by: Mellanox Technologies // NVIDIA Networking