Michael Dexter <editor@callfortesting.org> reported
a crash in FreeNAS, where the first argument to
clnt_bck_svccall() was no longer valid.
This argument is a pointer to the callback CLIENT
structure, which is free'd when the associated
NFSv4 ClientID is free'd.

This appears to have occurred because a callback
reply was still in the socket receive queue.

This patch acquires a reference count on the CLIENT
that is not CLNT_RELEASE()'d until the socket structure
is destroyed. This should guarantee that the CLIENT
structure is still valid when clnt_bck_svccall() is called.
It also adds a check for closed or closing to
clnt_bck_svccall() so that it will not process the callback
RPC reply message after the ClientID is free'd.

Tested for both Linux and FreeBSD NFSv4 clients
with/without delegations enabled with a printf()
for the CLIENT reference count when svc_vc_destroy()
is called.
It worked as expected. No leak of M_RPC malloc()s
have been detected.

Michael Dexter <editor@callfortesting.org>
will hopefully be able to test this patch as well.