RDMA/nes: Fix error handling issues

Fix issues found by static code analysis:

(1) Check if cm_node was successfully created for loopback connection.

(2) schedule_nes_timer() does not free up allocated memory after
    encountering an error.  There is a WARN_ON() for this condition.

(3) there is a cm_node->freed flag which is set but not used.

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Faisal Latif <faisal.latif@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>