knfsd: let nfsd manage timing out its own leases

Currently there's a race that can cause an oops in generic_setlease.

(In detail: nfsd, when it removes a lease, does so by calling
vfs_setlease() with F_UNLCK and a pointer to the fl_flock field, which
in turn points to nfsd's existing lease; but the first thing the
setlease code does is call time_out_leases().  If the lease happens to
already be beyond the lease break time, that will free the lease and (in
nfsd's release_private callback) set fl_flock to NULL, leading to a NULL
deference soon after in vfs_setlease().)

There are probably other things to fix here too, but it seems inherently
racy to allow either locks.c or nfsd to time out this lease.  Instead
just set the fl_break_time to 0 (preventing locks.c from ever timing out
this lock) and leave it up to nfsd's laundromat thread to deal with it.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/nfs4state.c

@@ -1354,8 +1354,12 @@ void nfsd_break_deleg_cb(struct file_lock *fl)
 	/* only place dl_time is set. protected by lock_kernel*/
 	dp->dl_time = get_seconds();
 
-	/* XXX need to merge NFSD_LEASE_TIME with fs/locks.c:lease_break_time */
-	fl->fl_break_time = jiffies + NFSD_LEASE_TIME * HZ;
+	/*
+	 * We don't want the locks code to timeout the lease for us;
+	 * we'll remove it ourself if the delegation isn't returned
+	 * in time.
+	 */
+	fl->fl_break_time = 0;
 
 	t = kthread_run(do_recall, dp, "%s", "nfs4_cb_recall");
 	if (IS_ERR(t)) {