inpcb: return ENOMEM if bind(2) fails to allocate lbgroup

[glebius]

Summary

The SO_REUSEPORT_LB isn't a standard option, neither ENOMEM is a specified
return code from bind(2), but it definitely is more appropriate than
EAGAIN or the masked ENOBUFS.

[markj]

sys/netinet/in_pcb.c

736

What's the purpose of this assertion? If you want to document that in_pcbinshash() can only fail for lbgroup sockets, IMO it would be better to somehow document that fact in in_pcbinshash().