SUNRPC: Clean up address type casts in rpcb_v4_register()

Clean up: Simplify rpcb_v4_register() and its helpers by moving the
details of sockaddr type casting to rpcb_v4_register()'s helper
functions.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>