client: skip putting inodes if map is empty

client: skip putting inodes if map is empty

The delay_put_inodes() will be called by the tick() periodically
per second, and when the _unmount() is waiting for the inode_map
to be cleared, if the delay_i_release is empty the _unmount() will
be woke up too frequent that the dump_cache() won't get any chance
to be called.

Fixes: https://tracker.ceph.com/issues/48707
Signed-off-by: Xiubo Li <xiubli@redhat.com>