selftests/bpf: Fix vmtest.sh -h to not require root

Set the exit trap only after argument parsing is done. This way argument
parse failure or `-h` will not require sudo.

Reasoning is that it's confusing that a help message would require root
access.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Müller <deso@posteo.net>
Link: https://lore.kernel.org/bpf/6a802aa37758e5a7e6aa5de294634f5518005e2b.1660064925.git.dxu@dxuuu.xyz