nvmecontrol: Have to truncate on all 32-bit architectures

armv7, powerpc, powerpcspe and i386 all lack 128-bit integer
types. Adjust the comment and #ifdef. I don't think we support nvme on
any of these other architectures at the moment, but it won't hurt to be
more precise.

Sponsored by: Netflix
Reviewed by: chuck
Differential Revision: https://reviews.freebsd.org/D44650