selftests/bpf: Add tests for bpf_task_from_vpid() kfunc

This patch adds test cases for bpf_task_from_vpid() kfunc.

task_kfunc_from_vpid_no_null_check is used to test the case where
the return value is not checked for NULL pointer.

test_task_from_vpid_current is used to test obtaining the
struct task_struct of the process in the pid namespace based on vpid.

test_task_from_vpid_invalid is used to test the case of invalid vpid.

test_task_from_vpid_current and test_task_from_vpid_invalid will run
in the new namespace.

Signed-off-by: Juntong Deng <juntong.deng@outlook.com>
Link: https://lore.kernel.org/r/AM6PR03MB5848F13435CD650AC4B7BD7099442@AM6PR03MB5848.eurprd03.prod.outlook.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>