nfs: fix build error in nfsroot with initconst

fix build error with latest kbuild adjustments to initconst.

The commit a447c093 ("vfs: Use
const for kernel parser table") changed:

    static match_table_t __initdata tokens = {
to
    static match_table_t __initconst tokens = {

But the missing const causes popwerpc to fail with latest
updates to __initconst like this:

fs/nfs/nfsroot.c:400: error: __setup_str_nfs_root_setup causes a section type conflict
fs/nfs/nfsroot.c:400: error: __setup_str_nfs_root_setup causes a section type conflict

The bug is only present with kbuild-next.
Following patch has been build tested.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfsroot.c

@@ -129,7 +129,7 @@ enum {
 	Opt_err
 };
 
-static match_table_t __initconst tokens = {
+static const match_table_t tokens __initconst = {
 	{Opt_port, "port=%u"},
 	{Opt_rsize, "rsize=%u"},
 	{Opt_wsize, "wsize=%u"},