xprtrdma: Add a "register_external" op for each memreg mode

There is very little common processing among the different external
memory registration functions. Have rpcrdma_create_chunks() call
the registration method directly. This removes a stack frame and a
switch statement from the external registration path.

Signed-off-by: Chuck Lever <chuck.lever@oracle.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>