net/smc: use smc_curs_copy() for SMC-D

SMC already provides a wrapper for atomic64 calls to be
architecture independent. Use this wrapper for SMC-D as well.

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