in_pcb: delay crfree() down into UMA dtor

inpcb lookups, which check inp_cred, work with pcbs that potentially went
through in_pcbfree(). So inp_cred should stay valid until SMR guarantees
its invisibility to lookups.

While here, put the whole inpcb destruction sequence of in_pcbfree(),
inpcb_dtor() and inpcb_fini() sequentially.

Submitted by: markj
Differential revision: https://reviews.freebsd.org/D33273