RDMA/cxgb3: Annotate locking assumptions

Tell sparse what the locking assumptions are for __flush_qp() such
that it does not complain about the locking operations inside that
function.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>