NFS: Clean-up: Refactor IP address sanity checks in NFS client

NFS and NFSv4 mounts can now share server address sanity checking.  And, it
provides an easy mechanism for adding IPv6 address checking at some later
point.

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>