SUNRPC: Refactor rpcb_register to make rpcbindv4 support easier

rpcbind version 4 registration will reuse part of rpcb_register, so just
split it out into a separate function now.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>