MFC r314509 and r314578.

r314509:
cxgbe/iw_cxgbe: Do not check the size of the memory region being
registered. T4/5/6 have no internal limit on this size. This is
probably a copy paste from the T3 iw_cxgb driver.

r314578:
cxgbe/iw_cxgbe: Implement sq/rq drain operation.

ULPs can set a qp's state to ERROR and then post a work request on the
sq and/or rq. When the reply for that work request comes back it is
guaranteed that all previous work requests posted on that queue have
been drained.

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