Note: The message may also be mb_map too small!
The panic indicates that the system ran out of virtual memory for network buffers (specifically, mbuf clusters). You can increase the amount of VM available for mbuf clusters by adding:
to your kernel config file, where n is a number in the range 512-4096, depending on the number of concurrent TCP connections you need to support. I would recommend trying 2048 - this should get rid of the panic completely. You can monitor the number of mbuf clusters allocated/in use on the system with netstat -m (see netstat(1)). The default value for NMBCLUSTERS is 512 + MAXUSERS * 16.