inpcb: fix a panic with SO_REUSEPORT_LB + connect(2) misuse

[glebius]

Summary

This combination doesn't make any sense. However, we shouldn't panic.
There are two options here: refuse connect(2) on a socket that has the
option set previously, or ignore (and clear) the option. We have chosen
the latter, because connect(2) is a SUS API that has a defined set of
error codes, none of which corresponds to "a socket has non-standard
incompatible socket option set". We don't want to extend connect(2) error
codes list to report a awkward misuse of a non-standard socket option.