rds: remove redundant variable 'sg_off'

Variable sg_off is assigned a value but it is never read, hence it is
redundant and can be removed.

Cleans up clang warning:
net/rds/message.c:373:2: warning: Value stored to 'sg_off' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>