libceph: support for sending notifies

Implement ceph_osdc_notify() for sending notifies.

Due to the fact that the current messenger can't do read-in into
pagelists (it can only do write-out from them), I had to go with a page
vector for a NOTIFY_COMPLETE payload, for now.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

include/linux/ceph/rados.h

@@ -476,6 +476,9 @@ struct ceph_osd_op {
 			__u8 op;	/* CEPH_OSD_WATCH_OP_* */
 			__le32 gen;     /* registration generation */
 		} __attribute__ ((packed)) watch;
+		struct {
+			__le64 cookie;
+		} __attribute__ ((packed)) notify;
 		struct {
 			__le64 offset, length;
 			__le64 src_offset;