Teach NFS_ROOT to use the new rpc_create API instead of the old two-call
API for creating an RPC transport.

Test plan:
Compile the kernel with the NFS client build-in, and set CONFIG_NFS_ROOT.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>