ceph: add dentry lease metric support

For dentry leases, only count the hit/miss info triggered from the vfs
calls. For the cases like request reply handling and ceph_trim_dentries,
ignore them.

For now, these are only viewable using debugfs. Future patches will
allow the client to send the stats to the MDS.

The output looks like:

item          total           miss            hit
-------------------------------------------------
d_lease       11              7               141

URL: https://tracker.ceph.com/issues/43215
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
diff --git a/fs/ceph/mds_client.h b/fs/ceph/mds_client.h
index 903d9ed..605995a 100644
--- a/fs/ceph/mds_client.h
+++ b/fs/ceph/mds_client.h
@@ -16,6 +16,8 @@
 #include <linux/ceph/mdsmap.h>
 #include <linux/ceph/auth.h>
 
+#include "metric.h"
+
 /* The first 8 bits are reserved for old ceph releases */
 enum ceph_feature_type {
 	CEPHFS_FEATURE_MIMIC = 8,
@@ -454,6 +456,8 @@ struct ceph_mds_client {
 	struct list_head  dentry_leases;     /* fifo list */
 	struct list_head  dentry_dir_leases; /* lru list */
 
+	struct ceph_client_metric metric;
+
 	spinlock_t		snapid_map_lock;
 	struct rb_root		snapid_map_tree;
 	struct list_head	snapid_map_lru;