nfsd: clean up grace period on early exit

If nfsd was shut down before the grace period ended, we could end up
with a freed object still on grace_list.  Thanks to Jeff Moyer for
reporting the resulting list corruption warnings.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Tested-by: Jeff Moyer <jmoyer@redhat.com>