rds: ib: add the static type to the variables

The variables rds_ib_mr_1m_pool_size and rds_ib_mr_8k_pool_size
are used only in the ib.c file. As such, the static type is
added to limit them in this file.

Cc: Joe Jin <joe.jin@oracle.com>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>