RDMA/nes: Check cqp_avail_reqs is empty after locking the list

Between the first empty list check and locking the list, the list can
change.  Check it again after it is locked to make sure the list is
still not empty.

Signed-off-by: Faisal Latif <faisal.latif@intel.com>
Signed-off-by: Chien Tung <chien.tin.tung@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>