mount.nfs4: Backgrounding mount broken with NFS versions <4

When the NFS version isn't specified in the mount options, mount.nfs
attempts V4 first and appends 'vers=4' to the extra_options string in
the mount options.  If the server isn't immediately reachable, this
attempt fails.  However, if the background option is specified and the
server comes up later on, the extra_options are used again for all
further attempts and thus they fail if the server only supports
vers<4.

Fix this by only amending extra_options on a successful vers=4 mount.

This is now Debian bug #690181 and has apparently been around for
ages.

Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Wolfram Gloger <bugzilla1@malloc.de>
Signed-off-by: Steve Dickson <steved@redhat.com>