pidfs: record the coredump on the dumping thread's pid too

If a thread-group coredumps only the thread-group leader pidfd will
return coredump information. A pidfd for the thread that took the fatal
signal cannot be used to retrieve it.

Record both the thread-group leader and the specific thread that took
the signal and register both in pidfs. Mark both the thread-group leader
and the specific thread with the coredump information so retrieval works
for both pidfds.

Now that both SO_PEERPIDFD and SO_PEERPIDFD_THREAD are available it's
easy to get the coredump information for the specific thread.

Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@futurfusion.io>
Signed-off-by: Andrew Halaney <andrew@amutable.com>
Link: https://patch.msgid.link/20261005-work-unix-passpidfd-v4-9-350183b6e02a@amutable.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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