lib/idmapping.c: get_map_ranges(): Merge two input checks into a simpler one

lib/idmapping.c: get_map_ranges(): Merge two input checks into a simpler one

Previously, we were performing the following two checks:

-       if (ranges != ((argc + 2) / 3)) {
-       if ((ranges * 3) > argc) {

Let's draw a table of the possible input that would pass the first check:

argc:	0 1 2 3 4 5 6 7 8 9
rng:	0 1 1 1 2 2 2 3 3 3
a+2/3*3:0 3 3 3 6 6 6 9 9 9	<-- this is  roundup(argc, 3);
a+2/3:	0 1 1 1 2 2 2 3 3 3	<-- this is  roundup(argc, 3) / 3;
rng*3:	0 3 3 3 6 6 6 9 9 9

From those, let's extract those that would also pass the second check:

argc:	0     3     6     9
rng:	0     1     2     3
rng*3:	0     3     6     9

We can see that there's a simple check for this input:

+       if (ranges * 3 != argc) {

As a sanity check, let's draw a table of the acceptable input with that
check:

rng:	0     1     2     3
rng*3:	0     3     6     9
argc:	0     3     6     9

Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>