SUNRPC: prevent task_cleanup running on freed xprt

We saw a report of a NULL dereference in xprt_autoclose:

	https://bugzilla.redhat.com/show_bug.cgi?id=611938



This appears to be the result of an xprt's task_cleanup running after
the xprt is destroyed.  Nothing in the current code appears to prevent
that.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>