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.
Tested by: Illias A. Marinos, Herbert Poeckl
Reviewed by: jhb
MFC after: 2 weeks
13870d5d7b