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

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