ceph: ensure rename target dentry fails revalidation

This works around a bug in vfs_rename_dir() that rehashes the target
dentry.  Ensure such dentries always fail revalidation by timing out the
dentry lease and kicking it out of the current directory lease gen.

This can be reverted when the vfs bug is fixed.

Signed-off-by: Sage Weil <sage@newdream.net>