RDS: IB: Re-organise ibmr code

No functional changes. This is in preperation towards adding
fastreg memory resgitration support.

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>