ceph: optimize memory usage

In current code, regular file and directory use same struct
ceph_file_info to store fs specific data so the struct has to
include some fields which are only used for directory
(e.g., readdir related info), when having plenty of regular files,
it will lead to memory waste.

This patch introduces dedicated ceph_dir_file_info cache for
readdir related thins. So that regular file does not include those
unused fields anymore.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/super.h

@@ -671,6 +671,10 @@ struct ceph_file_info {
 
 	spinlock_t rw_contexts_lock;
 	struct list_head rw_contexts;
+};
+
+struct ceph_dir_file_info {
+	struct ceph_file_info file_info;
 
 	/* readdir: position within the dir */
 	u32 frag;