posixshm_test.c: tautology in test

[rlibby]

Summary

gcc -Wtype-limits complained about unsigned >= 0. Cast to signed so
that it might check for underflow.

---

Alternately, we might just delete the checks.

Test Plan

env MAKEOBJDIRPREFIX=/usr/obj/gcc6 CROSS_TOOLCHAIN=amd64-gcc make buildworld
env MAKEOBJDIRPREFIX=/usr/obj/clang make buildworld

... I'll look into actually running this test too.

[kib]

The in-kernel source for the sysctl is also unsigned.
I think that the checks can be safely dropped.

[markj]

In D27564#615982, @kib wrote:

The in-kernel source for the sysctl is also unsigned.
I think that the checks can be safely dropped.

I agree. I can't remember why I added the checks.

[rlibby]

feedback: just remove the checks