Unset the gss kernel state when gssd exits

When gssd exits it leaves the kernel state set by
gssd_syscall(). nfsd sees this and waits endlessly
in an unkillable state for gssd to come back. If you
had acidentally started gssd then stopped it, then
started nfsd you'd be in a bad way until you either
restarted gssd or rebooted the system. This change
fixes that by setting the kernel state to "" when
gssd exits.
Reviewed by: rmacklem
MFC after: 1 week
Sponsored by: iXsystems