ceph: initialize inode before instantiating dentry

commit b18825a7c8 (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: #6930
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>