netrom: fix invalid use of sizeof in nr_recvmsg()

sizeof() when applied to a pointer typed expression gives the size of the
pointer, not that of the pointed data.
Introduced by commit 3ce5ef(netrom: fix info leak via msg_name in nr_recvmsg)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>