Merge patch series "pidfd: improve uapi when task isn't found"

Christian Brauner <brauner@kernel.org> says:

We currently report EINVAL whenever a struct pid has no tasked attached
anymore thereby conflating two concepts:

(1) The task has already been reaped.
(2) The caller requested a pidfd for a thread-group leader but the pid
    actually references a struct pid that isn't used as a thread-group
    leader.

This is causing issues for non-threaded workloads as in [1] where they
expect ESRCH to be reported, not EINVAL. I think that's a very resonable
assumption.

This patch tries to allow userspace to distinguish between (1) and (2).
This is racy of course but that shouldn't matter.

* patches from https://lore.kernel.org/r/20250403-work-pidfd-fixes-v1-0-a123b6ed6716@kernel.org:
  selftest/pidfd: add test for thread-group leader pidfd open for thread
  pidfd: improve uapi when task isn't found
  pidfd: remove unneeded NULL check from pidfd_prepare()
  selftests/pidfd: adapt to recent changes

Link: https://lore.kernel.org/r/20250403-work-pidfd-fixes-v1-0-a123b6ed6716@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>