pcb: Move an assignment into in_pcbdisconnect()

All callers of in_pcbdisconnect() clear the local address, so let's just
do that in the function itself.

Note that the inp's local address is not a parameter to the inp hash
functions. No functional change intended.

Reviewed by: glebius
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D38362