ceph: initialize inode before instantiating dentry

commit b18825a7 (Put a small type field into struct dentry::d_flags)
put a type field into struct dentry::d_flags. __d_instantiate() set the
field by checking inode->i_mode. So we should initialize inode before
instantiating dentry when handling mds reply.

Fixes: http://tracker.ceph.com/issues/6930


Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Sage Weil <sage@inktank.com>