NFS: Revert NFSROOT default mount options

Marek Belisko <marek.belisko@gmail.com> reports that recent attempts
to fix regressions in NFSROOT have broken his configuration:

> After update from 2.6.38-rc8 to 2.6.38 is mounting rootfs over nfs not possible.
> Log:
> VFS: Mounted root (nfs filesystem) on device 0:14.
> Freeing init memory: 132K
> nfs: server 10.146.1.21 not responding, still trying
> nfs: server 10.146.1.21 not responding, still trying
>
> This is never ending. I make short bisect (not too much commits
> between versions)
> and bad commit was reported: 53d47375
>
> NFS: NFSROOT should default to "proto=udp"
>
> I've tested on mini2440 board (DM9000, static IP).
> Is there some missing option or something else to be checked?

An examination of a network trace captured during the failure shows
that the mount is actually succeeding, but that the client is not
seeing READ replies larger than 16KB.  This could be a local packet
filtering issue on the client, but we didn't troubleshoot this
further because of the reported "git bisect" result.

Last fall we removed the ad hoc mount option parser in
fs/nfs/nfsroot.c in favor of using the main parser in fs/nfs/super.c
(see commit 56463e50 "NFS: Use super.c for NFSROOT mount option
parsing").  That commit changed the default NFSROOT mount options to
be the same as those employed by user space mounts.

As it turns out, these new default mount options are not tolerated by
many embedded systems.  So far these problems have been due to
specific behavior of certain embedded NICs.  The NFS community does
not have such hardware on hand for running tests.

Commit 53d47375 recently introduced a clean way to specify default
mount options for NFSROOT, so we can now easily restore the
traditional defaults for NFSROOT:

   vers=2,udp,rsize=4096,wsize=4096

This should revert the new default NFSROOT mount options introduced
with commit 56463e50.

Tested-by: Marek Belisto <marek.belisto@open-nandra.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>