IB/core: Add support for draining IB_POLL_DIRECT completion queues

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Steve Wise <swise@opengridcomputing.com>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>