umount: use correct return value for is_vers4.

umount: use correct return value for is_vers4.

is_vers4 in mount_libmount.c is based on nfs_umount_is_vers4
in nfsumount.c, except the return values are reversed.
The result of this is:
 - a MOUNT_UMNT call is not sent when an NFSv3 or NFSv2
   filesystem is unmounted
 - a MOUNT_UMNT call *is* sent when and 'nfs4' filesystem
   is unmounted (but not when an 'nfs -o vers=4 filesystem
   is unmounted, as that is checked elsewhere).

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Steve Dickson <steved@redhat.com>