selftests/bpf: add a test for iter/task_vma for short-lived processes

selftests/bpf: add a test for iter/task_vma for short-lived processes

When a task iterator traverses vma(s), it is possible task->mm might
become invalid in the middle of traversal and this may cause kernel
misbehave (e.g., crash)

This test case creates iterators repeatedly and forks short-lived
processes in the background to detect this bug.  The test will last
for 3 seconds to get the chance to trigger the issue.

Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221216221855.4122288-3-kuifeng@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>