ceph: periodically trim stale dentries

Previous commit make VFS delete stale dentry when last reference is
dropped. Lease also can become invalid when corresponding dentry has
no reference. This patch make cephfs periodically scan lease list,
delete corresponding dentry if lease is invalid.

There are two types of lease, dentry lease and dir lease. dentry lease
has life time and applies to singe dentry. Dentry lease is added to tail
of a list when it's updated, leases at front of the list will expire
first. Dir lease is CEPH_CAP_FILE_SHARED on directory inode, it applies
to all dentries in the directory. Dentries have dir leases are added to
another list. Dentries in the list are periodically checked in a round
robin manner.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>