ceph: unify cap flush and snapcap flush

This patch includes following changes
- Assign flush tid to snapcap flush
- Remove session's s_cap_snaps_flushing list. Add inode to session's
  s_cap_flushing list instead. Inode is removed from the list when
  there is no pending snapcap flush or cap flush.
- make __kick_flushing_caps() re-send both snapcap flushes and cap
  flushes.

Signed-off-by: Yan, Zheng <zyan@redhat.com>

fs/ceph/mds_client.h

@@ -152,7 +152,6 @@ struct ceph_mds_session {
 
 	/* protected by mutex */
 	struct list_head  s_cap_flushing;     /* inodes w/ flushing caps */
-	struct list_head  s_cap_snaps_flushing;
 	unsigned long     s_renew_requested; /* last time we sent a renew req */
 	u64               s_renew_seq;