net/smc: Avoid overwriting the copies of clcsock callback functions

In the Linux kernel, the following vulnerability has been resolved:

net/smc: Avoid overwriting the copies of clcsock callback functions

The callback functions of clcsock will be saved and replaced during

the fallback. But if the fallback happens more than once, then the

copies of these callback functions will be overwritten incorrectly,

resulting in a loop call issue:

clcsk->sk_error_report

|- smc_fback_error_report() <------------------------------|

|- smc_fback_forward_wakeup() | (loop)

|- clcsock_callback() (incorrectly overwritten) |

|- smc->clcsk_error_report() ------------------|

So this patch fixes the issue by saving these function pointers only

once in the fallback and avoiding overwriting.