MFC r265232

Fix a panic caused by doing "ifconfig -am" while a lagg is being destroyed.
The thread that is destroying the lagg has already set sc->sc_psc=NULL when
the "ifconfig -am" thread gets to lacp_req().  It tries to dereference
sc->sc_psc and panics.  The solution is for lacp_req() to check the value of
sc->sc_psc.  If NULL, harmlessly return an lacp_opreq structure full of
zeros.  Full details in GNATS.

PR:   189003