rds: Fix build on powerpc.

As reported by Stephen Rothwell.

> Today's linux-next build (powerpc allyesconfig) failed like this:
>
> net/rds/cong.c: In function 'rds_cong_set_bit':
> net/rds/cong.c:284: error: implicit declaration of function 'generic___set_le_bit'
> net/rds/cong.c: In function 'rds_cong_clear_bit':
> net/rds/cong.c:298: error: implicit declaration of function 'generic___clear_le_bit'
> net/rds/cong.c: In function 'rds_cong_test_bit':
> net/rds/cong.c:309: error: implicit declaration of function 'generic_test_le_bit'

Signed-off-by: David S. Miller <davem@davemloft.net>