nfsd4: require version 4 when enabling or disabling minorversion

The current code will allow silly things like:

	echo "+2 +3 +4 +7.1">/proc/fs/nfsd/versions

Reported-by: Fan Chaoting <fanchaoting@cn.fujitsu.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>