net: smc: fix spurious error message from __sock_release()

Commit 67f562e3 ("net/smc: transfer fasync_list in case of fallback")
leaves the socket's fasync list pointer within a container socket as well.
When the latter is destroyed, '__sock_release()' warns about its non-empty
fasync list, which is a dangling pointer to previously freed fasync list
of an underlying TCP socket. Fix this spurious warning by nullifying
fasync list of a container socket.

Fixes: 67f562e3

 ("net/smc: transfer fasync_list in case of fallback")
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>