net/smc: add error handling for get_user()

For security reasons the return code of get_user() should always be
checked.

Fixes: 01d2f7e2 ("net/smc: sockopts TCP_NODELAY and TCP_CORK")
Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>