selftests/bpf: Fix bpf_cookie and find_vma in nested VM

selftests/bpf: Fix bpf_cookie and find_vma in nested VM

bpf_cookie and find_vma are flaky in nested VMs, which is used by some CI
systems. It turns out these failures are caused by unreliable perf event
in nested VM. Fix these by:

  1. Use PERF_COUNT_SW_CPU_CLOCK in find_vma;
  2. Increase sample_freq in bpf_cookie.

Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240604070700.3032142-1-song@kernel.org