tools/testing/selftests/sysctl/sysctl.sh: ignore diff output on verify_diff_w()

When verify_diff_w() is used we care about the result, not the verbose
output, and although we use -q, that still gives us a chatty message
about if the files differ or not.  Since verify_diff_w() uses stdinput
the chatty message says whether or not "-" matches the target file, and
this just seems rather odd.  Better to just ignore that messsage all
together, what we really care about i sthe results, the return value and
we check for that.

Link: http://lkml.kernel.org/r/20190320222831.8243-4-mcgrof@kernel.org


Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Eric Sandeen <sandeen@redhat.com>
Cc: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>