Fix various compiler warnings about narrowing or signedness changes.

This is mostly done by inserting explicit casts, though one function got
its return type changed.

Bug: chromium:1292951
Change-Id: Ia82a34db88dcc21d235c573b5ca581482e8f8ed8
Reviewed-on: https://chromium-review.googlesource.com/c/linux-syscall-support/+/3736690
Reviewed-by: Mike Frysinger <vapier@chromium.org>