MFC: r244370

MFC: r244370
Piete.Brooks at cl.cam.ac.uk reported via email a crash which was
caused by use of an invalid kgss_gssd_handle during an upcall to
the gssd daemon when it has exited. This patch seems to avoid the
crashes by holding a reference count on the kgss_gssd_handle until
the upcall is done. It also adds a new mutex kgss_gssd_lock used to
make manipulation of kgss_gssd_handle SMP safe.

git-svn-id: svn://svn.freebsd.org/base/stable/9@245018 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f