Unnecessary dependecy between nfsv3 and flexfiles modules

["Mkrtchyan, Tigran" <tigran.mkrtchyan@xxxxxxx>]

Dear NFS developers,

I have noticed an unnecessary dependency between the pNFS flexfiles layout driver and NFSv3 modules:

```Kconfig 
config PNFS_FLEXFILE_LAYOUT
        tristate
        depends on NFS_V4_1 && NFS_V3
        default NFS_V4

```

Thus, by disabling v3 (as one might do if only v4.1+ is needed), the flexfiles
layout driver is not available anymore.

It looks like, that flexfiles layout doesn't really depend on NFSv3 code and if I remove
this dependency, that I can build and run a kernel with flexfiles layout driver, but without
NFSv3 support:


```
[root@os-46-nfs-devel ~]# lsmod
Module                  Size  Used by
nfs_layout_flexfiles    61440  1
nfsv4                1200128  2 nfs_layout_flexfiles
dns_resolver           12288  1 nfsv4
[root@os-46-nfs-devel ~]#
```

But might be there is a non-obvious dependency that I miss. Otherwise I happy to fix that :)

Best regards,
   Tigran.

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