mount.nfs does not downgrade NFS version when connecting to dual-stack NFS server

[Impact]
 When all the following exist
   - mounting a server using hostname.
   - the hostname resolves to both an ipv4 and ipv6 address
   - the nfs server only supports nfsv3
 Then.
 * The nfs client will not fall back to nfsv3, and will be unable to mount
   the share.
 * The following errors are printed in the logs.
"
mount: no type was given - I'll assume nfs because of the colon
mount.nfs: timeout set for Tue Nov 11 14:25:34 2014
mount.nfs: trying text-based options 'vers=4,addr=x.x.x.x,clientaddr=x.x.x.x'
mount.nfs: mount(2): Protocol not supported
mount.nfs: trying text-based options 'vers=4,addr=xxxx:xxxx::xxxx,clientaddr=xxxx:xxxx::xxxx'
mount.nfs: mount(2): No route to host
mount.nfs: trying text-based options 'vers=4,addr=x.x.x.x,clientaddr=x.x.x.x'
mount.nfs: mount(2): Protocol not supported
mount.nfs: trying text-based options 'vers=4,addr=xxxx:xxxx::xxxx,clientaddr=xxxx:xxxx::xxxx'
mount.nfs: mount(2): No route to host
"

[Test Case]

 * Restrict to nfsv3 on Server by setting RPCNFSDOPTS="--no-nfs-version 4" in /etc/default/nfs-kernel-server
 * Enable dns resolution for both ipv4 and ipv6 of the server.
 * Export a directory from the server (export -a)
 * Attempt to mount the nfs share from client

[Regression Potential]
 * Upstream backport that still exists upstream.

[Other Info]
 * Fix already exists in 2.6+ which means >trusty already have the fix.

__________________________________________________________________________
If you attempt to mount a share from a server that is dual stack (i.e. has both an A and a AAAA record in DNS) and NFSv3-only, mount.nfs goes into an infinite loop of retrying an NFS-v4 mount:

mount nfs-v3-server:/path /mnt -v
mount: no type was given - I'll assume nfs because of the colon
mount.nfs: timeout set for Tue Nov 11 14:25:34 2014
mount.nfs: trying text-based options 'vers=4,addr=x.x.x.x,clientaddr=x.x.x.x'
mount.nfs: mount(2): Protocol not supported
mount.nfs: trying text-based options 'vers=4,addr=xxxx:xxxx::xxxx,clientaddr=xxxx:xxxx::xxxx'
mount.nfs: mount(2): No route to host
mount.nfs: trying text-based options 'vers=4,addr=x.x.x.x,clientaddr=x.x.x.x'
mount.nfs: mount(2): Protocol not supported
mount.nfs: trying text-based options 'vers=4,addr=xxxx:xxxx::xxxx,clientaddr=xxxx:xxxx::xxxx'
mount.nfs: mount(2): No route to host

If we "hide" the AAAA record by setting an override in /etc/hosts, then mount.nfs correctly retries with NFSv3:

mount -t nfs nfs-v3-server:/path /mnt -v
mount.nfs: timeout set for Tue Nov 11 15:01:35 2014
mount.nfs: trying text-based options 'vers=4,addr=x.x.x.x,clientaddr=x.x.x.x'
mount.nfs: mount(2): Protocol not supported
mount.nfs: trying text-based options 'addr=x.x.x.x'
mount.nfs: prog 100003, trying vers=3, prot=6
mount.nfs: trying x.x.x.x prog 100003 vers 3 prot TCP port 2049
mount.nfs: prog 100005, trying vers=3, prot=17
mount.nfs: trying x.x.x.x prog 100005 vers 3 prot UDP port 635
nfs-v3-server:/path on /mnt type nfs (rw)

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: nfs-common 1:1.2.5-3ubuntu3.1
ProcVersionSignature: Ubuntu 3.2.0-70.105-generic 3.2.63
Uname: Linux 3.2.0-70-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.8
Architecture: amd64
Date: Tue Nov 11 15:56:25 2014
InstallationMedia: Ubuntu-Server 10.04.2 LTS "Lucid Lynx" - Release amd64 (20110211.1)
MarkForUpload: True
ProcEnviron:
 SHELL=/usr/bin/ksh
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US
SourcePackage: nfs-utils
UpgradeStatus: Upgraded to precise on 2013-10-07 (400 days ago)