RDS: add __printf format attribute to error reporting functions

This is helpful to detect at compile-time errors related to format
strings.

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>