net/smc: put slot when connection is killed

To get a send slot smc_wr_tx_get_free_slot() is called, which might
wait for a free slot. When smc_wr_tx_get_free_slot() returns there is a
check if the connection was killed in the meantime. In that case don't
only return an error, but also put back the free slot.

Fixes: b2900980 ("net/smc: cancel send and receive for terminated socket")
Reviewed-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>