ceph: keep reference to parent inode on ceph_dentry

When creating a new dentry we now hold a reference to the parent
inode in the ceph_dentry.  This is required due to the new RCU
changes from 949854d0

, which set dentry->d_parent to NULL in d_kill before
calling the ->release() callback.  If/when that behavior is changed, we can
revert this hack.

Signed-off-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Sage Weil <sage@newdream.net>