inpcb: Release the inpcb cred reference before freeing the structure

Now that the inp_cred pointer is accessed only while the inpcb lock is
held, we can avoid deferring a crfree() call when freeing an inpcb.

This fixes a problem introduced when inpcb hash tables started being
synchronized with SMR: the credential reference previously could not be
released until all lockless readers have drained, and there is no
mechanism to explicitly purge cached, freed UMA items. Thus, ucred
references could linger indefinitely, and since ucreds hold a jail
reference, the jail would linger indefinitely as well. This manifests
as jails getting stuck in the DYING state.

Discussed with: glebius
Tested by: glebius
Sponsored by: Klara, Inc.
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D38573