libceph: initialize last_linger_id with a large integer

osdc->last_linger_id is a counter for lreq->linger_id, which is used
for watch cookies.  Starting with a large integer should ease the task
of telling apart kernel and userspace clients.

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

include/linux/ceph/osd_client.h

@@ -258,6 +258,8 @@ struct ceph_watch_item {
 	struct ceph_entity_addr addr;
 };
 
+#define CEPH_LINGER_ID_START	0xffff000000000000ULL
+
 struct ceph_osd_client {
 	struct ceph_client     *client;