bpf, selftests: Fix spelling mistake "tained" -> "tainted"

There appears to be a spelling mistake in a bpf test message. Fix it.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211217182400.39296-1-colin.i.king@gmail.com