NFS: Avoid warnings when CONFIG_NFS_V4=n

Avoid the following warnings when CONFIG_NFS_V4=n:

	fs/nfs/sysctl.c:19: warning: unused variable `nfs_set_port_max'
	fs/nfs/sysctl.c:18: warning: unused variable `nfs_set_port_min'

by making those variables contingent on NFSv4 being configured.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>