selftests/bpf: add __sink() macro to fake variable consumption

Add __sink(expr) macro that forces compiler to believe that passed in
expression is both read and written. It used a simple embedded asm for
this. This is useful in a lot of tests where we assign value to some variable
to trigger some action, but later don't read variable, causing compiler
to complain (if corresponding compiler warnings are turned on, which
we'll do in the next patch).

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230309054015.4068562-3-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/bpf_misc.h

@@ -76,6 +76,9 @@
 #define FUNC_REG_ARG_CNT 5
 #endif
 
+/* make it look to compiler like value is read and written */
+#define __sink(expr) asm volatile("" : "+g"(expr))
+
 struct bpf_iter_num;
 
 extern int bpf_iter_num_new(struct bpf_iter_num *it, int start, int end) __ksym;