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.

Reported-by:  <syzbot+4de3c0e8a263e1e499bc@syzkaller.appspotmail.com>
Fixes: 341adeec ("net/smc: Forward wakeup to smc socket waitqueue after fallback")
Link: https://lore.kernel.org/r/0000000000006d045e05d78776f6@google.com


Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>