bpf: Fix -Wshadow warnings

There are thousands of warnings about one macro in a W=2 build:

  include/linux/filter.h:561:6: warning: declaration of 'ret' shadows a previous local [-Wshadow]

Prefix all the locals in that macro with __ to avoid most of
these warnings.

Fixes: 492ecee ("bpf: enable program stats")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201026162110.3710415-1-arnd@kernel.org

Author: arndb

include/linux/filter.h

@@ -558,21 +558,21 @@ struct sk_filter {
 DECLARE_STATIC_KEY_FALSE(bpf_stats_enabled_key);
 
 #define __BPF_PROG_RUN(prog, ctx, dfunc)	({			\
-	u32 ret;							\
+	u32 __ret;							\
 	cant_migrate();							\
 	if (static_branch_unlikely(&bpf_stats_enabled_key)) {		\
-		struct bpf_prog_stats *stats;				\
-		u64 start = sched_clock();				\
-		ret = dfunc(ctx, (prog)->insnsi, (prog)->bpf_func);	\
-		stats = this_cpu_ptr(prog->aux->stats);			\
-		u64_stats_update_begin(&stats->syncp);			\
-		stats->cnt++;						\
-		stats->nsecs += sched_clock() - start;			\
-		u64_stats_update_end(&stats->syncp);			\
+		struct bpf_prog_stats *__stats;				\
+		u64 __start = sched_clock();				\
+		__ret = dfunc(ctx, (prog)->insnsi, (prog)->bpf_func);	\
+		__stats = this_cpu_ptr(prog->aux->stats);		\
+		u64_stats_update_begin(&__stats->syncp);		\
+		__stats->cnt++;						\
+		__stats->nsecs += sched_clock() - __start;		\
+		u64_stats_update_end(&__stats->syncp);			\
 	} else {							\
-		ret = dfunc(ctx, (prog)->insnsi, (prog)->bpf_func);	\
+		__ret = dfunc(ctx, (prog)->insnsi, (prog)->bpf_func);	\
 	}								\
-	ret; })
+	__ret; })
 
 #define BPF_PROG_RUN(prog, ctx)						\
 	__BPF_PROG_RUN(prog, ctx, bpf_dispatcher_nop_func)