selftests/bpf: Fix vmtest.sh getopts optstring

selftests/bpf: Fix vmtest.sh getopts optstring

Before, you could see the following errors:

  $ ./vmtest.sh -j
  ./vmtest.sh: option requires an argument -- j
  ./vmtest.sh: line 357: OPTARG: unbound variable

  $ ./vmtest.sh -z
  ./vmtest.sh: illegal option -- z
  ./vmtest.sh: line 357: OPTARG: unbound variable

Fix by adding ':' as first character of optstring. Reason is that getopts
requires ':' as the first character for OPTARG to be set in the `?` and `:`
error cases.

Note that the ':' as the first character of the optstring switches getopts
to silent mode. The desire to run in this mode seems to have been there all
along, as the script takes care of reporting errors.

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/0f93b56198328b6b4da7b4cf4662d05c3edb5fd2.1660064925.git.dxu@dxuuu.xyz