net: rds: use memset to optimize the recv

The function rds_inc_init is in recv process. To use memset can optimize
the function rds_inc_init.
The test result:

     Before:
     1) + 24.950 us   |        rds_inc_init [rds]();
     After:
     1) + 10.990 us   |        rds_inc_init [rds]();

Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>