selftests/bpf: Make sure mount directory exists

The mount directory for the selftests cgroup tree might
not exist so let's make sure it does exist by creating
it ourselves if it doesn't exist.

Signed-off-by: Daan De Meyer <daan.j.demeyer@gmail.com>
Link: https://lore.kernel.org/r/20231011185113.140426-9-daan.j.demeyer@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>