selftests/coredump: fix userspace coredump client detection

selftests/coredump: fix userspace coredump client detection

PIDFD_INFO_COREDUMP is only retrievable until the task has exited. After
it has exited task->mm is NULL. So if the task didn't actually coredump
we can't retrieve it's dumpability settings anymore. Only if the task
did coredump will we have stashed the coredump information in the
respective struct pid.

Link: https://patch.msgid.link/20251028-work-coredump-signal-v1-15-ca449b7b7aa0@kernel.org
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>