xprtrdma: Add "init MRs" memreg op

This method is used when setting up a new transport instance to
create a pool of Memory Region objects that will be used to register
memory during operation.

Memory Regions are not needed for "physical" registration, since
->prepare and ->release are no-ops for that mode.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Tested-by: Devesh Sharma <Devesh.Sharma@Emulex.Com>
Tested-by: Meghana Cheripady <Meghana.Cheripady@Emulex.Com>
Tested-by: Veeresh U. Kokatnur <veereshuk@chelsio.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>