nfs: stash nfs_subversion reference into nfs_mount_info

That will allow to get rid of passing those references around in
quite a few places.  Moreover, that will allow to merge xdev and
remote file_system_type.

Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/nfs4super.c

@@ -248,6 +248,7 @@ static struct dentry *nfs4_referral_mount(struct file_system_type *fs_type,
 		.fill_super = nfs_fill_super,
 		.set_security = nfs_clone_sb_security,
 		.cloned = data,
+		.nfs_mod = &nfs_v4,
 	};
 	struct dentry *res;