RDMA/cxgb4: Reset wait condition atomically

The driver was never really waiting for RDMA_WR/FINI completions
because the condition variable used to determine if the completion
happened was never reset, and this condition variable is reused for
both connection setup and teardown.  This causes various driver
crashes under heavy loads due to releasing resources too early.

The fix is to use atomic bits to correctly reset the condition
immediately after the completion is detected.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>