Always use atomic_fetchadd() when updating per-user accounting values.

[jhb]

Summary

This avoids re-reading a variable after it has been updated via an
atomic op. It is just a cosmetic cleanup as the read value was only
used to control a diagnostic printf that should rarely occur (if ever).

Test Plan

• have had a variant of this patch around for several years and have booted amd64 and i386 VMs with it. OTOH, I have never seen this printf fire. It's been a low priority to ever get this reviewed but I noticed it today and figured might as well either get it in or discard it.

[kib]

sys/kern/kern_resource.c

1397 ↗

(On Diff #37527)

You can write '} else if (new < 0) {' to reduce indent.