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

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.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D49153