newuidmap,newgidmap: Correct the range size sanity check in get_map_ranges

The number of ranges should be the ceiling of the number of arguments divided
by three.

Without this fix newuidmap and newgidmap always report and error and fail,
which is very much not what we want.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>