smc: improve smc_clc_send_decline() error handling

Let smc_clc_send_decline() return with an error, if the amount
sent is smaller than the length of an smc decline message.

Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>