linuxkpi compiler.h: avoid gcc -Wunused-value in dummy expressions

It looks like the acquire and release macros are for the consumption
of static analysis tools and have no semantic effect. Transform the
definitions from constant expressions to empty statements in order to
avoid -Wunused-value from gcc.

Likewise avoid future warnings for __chk_{user,io}_ptr, but with a cast
to void, because it looks like some linux kernel code may use those in
expression contexts.

Reviewed by: hselasky, markj
Approved by: markj (mentor)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D11695