usnic: sendto() can return EPERM if we send too fast

If we send too fast, sendto() can run out of resources and return
EPERM.  So delay a little and try again.

opal/mca/btl/usnic/btl_usnic_cagent.c

@@ -298,6 +298,10 @@ static void agent_sendto(int fd, char *buffer, ssize_t numbytes,
         } else if (rc < 0) {
             if (errno == EAGAIN || errno == EINTR) {
                 continue;
+            } else if (errno == EPERM) {
+                // We're sending too fast
+                usleep(5);
+                continue;
             }
 
             char *msg;