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: 492ecee8

 ("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