net/smc: do not schedule tx_work in SMC_CLOSED state

The setsockopts options TCP_NODELAY and TCP_CORK may schedule the
tx worker. Make sure the socket is not yet moved into SMC_CLOSED
state (for instance by a shutdown SHUT_RDWR call).

Reported-by:  <syzbot+92209502e7aab127c75f@syzkaller.appspotmail.com>
Reported-by:  <syzbot+b972214bb803a343f4fe@syzkaller.appspotmail.com>
Fixes: 01d2f7e2 ("net/smc: sockopts TCP_NODELAY and TCP_CORK")
Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>