ceph: always initialize the dentry in open_root_dentry()

When open_root_dentry() gets a dentry via d_obtain_alias() it does
not get initialized.  If the dentry obtained came from the cache,
this is OK.  But if not, the result is an improperly initialized
dentry.

To fix this, call ceph_init_dentry() regardless of which path
produced the dentry.  That function returns immediately for a dentry
that is already initialized, it is safe to use either way.

(Credit to Sage, who suggested this fix.)

Signed-off-by: Alex Elder <aelder@sgi.com>