xprtrdma: Refactor rpcrdma_prepare_msg_sges()

Refactor: Replace spaghetti with code that makes it plain what needs
to be done for each rtype. This makes it easier to add features and
optimizations.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>