rds: remove redundant variable 'rds_ibdev'

Variable 'rds_ibdev' is being assigned but never used,
so can be removed.

fix this clang warning:
 net/rds/ib_send.c:762:24: warning: variable ‘rds_ibdev’ set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>