ceph: document what protects i_dirty_item and i_flushing_item

	struct rb_root i_caps;           /* cap list */

	struct ceph_cap *i_auth_cap;     /* authoritative cap, if any */

	unsigned i_dirty_caps, i_flushing_caps;     /* mask of dirtied fields */

	struct list_head i_dirty_item, i_flushing_item;

	struct list_head i_dirty_item, i_flushing_item; /* protected by

							 * mdsc->cap_dirty_lock

							 */

	/* we need to track cap writeback on a per-cap-bit basis, to allow

	 * overlapping, pipelined cap flushes to the mds.  we can probably

	 * reduce the tid to 8 bits if we're concerned about inode size. */