We create a lock-less link list for the currently
idle reusable smc_buf_desc.
When the 'used' filed mark to 0, it is added to
the lock-less linked list.
When a new connection is established, a suitable
element is obtained directly, which eliminates the
need for traversal and search, and does not require
locking resource.
A lock-free linked list is a linked list that uses
atomic operations to optimize the producer-consumer model.