RDS: IB: create struct rds_ib_fmr

Keep fmr related filed in its own struct. Fastreg MR structure
will be added to the union.

Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>