MFC: r291150

MFC: r291150
When the nfsd threads are terminated, the NFSv4 server state
(opens, locks, etc) is retained, which I believe is correct behaviour.
However, for NFSv4.1, the server also retained a reference to the xprt
(RPC transport socket structure) for the backchannel. This caused
svcpool_destroy() to not call SVC_DESTROY() for the xprt and allowed
a socket upcall to occur after the mutexes in the svcpool were destroyed,
causing a crash.
This patch fixes the code so that the backchannel xprt structure is
dereferenced just before svcpool_destroy() is called, so the code
does do an SVC_DESTROY() on the xprt, which shuts down the socket upcall.

git-svn-id: svn://svn.freebsd.org/base/stable/10@291869 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f