SUNRPC: Refactor a part of socket connect logic into a helper function

Finishing a socket connect is the same for IPv4 and IPv6, so split it out
into a helper.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Aurelien Charbon <aurelien.charbon@ext.bull.net>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>