selftests: devlink: fix the fd redirect in dummy_reporter_test

$number + > bash means redirect FD $number, e.g. commonly
used 2> redirects stderr (fd 2). The test uses 8192> to
write the number 8192 to a file, this results in:

  ./devlink.sh: line 499: 8192: Bad file descriptor

Oddly the test also papers over this issue by checking
for failure (expecting an error rather than success)
so it passes, anyway.

Fixes: ff18176a

 ("selftests: Add a test of large binary to devlink health test")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>