Merge patch series "coredump: cleanups & pidfd extension"

Christian Brauner <brauner@kernel.org> says:

The recent changes to rework coredump handling to rely on unix sockets
are in the process of being used in systemd. Yu reported on shortcoming
nameling that the signal causing the coredump was available before the
crashing process was reaped.

The previous systemd coredump container interface requires the coredump
file descriptor, and basic information including the signal number to be
sent to the container. This means we need to have the signal number
available before sending the coredump to the container.

In general, the extension makes sense and fits with the rest of the
coredump information.

In addition to this extension this fixes a bunch of the tests that were
failing and reworks the publication mechanism for exit and coredump info
retrievable via the pidfd ioctl.

* patches from https://patch.msgid.link/20251028-work-coredump-signal-v1-0-ca449b7b7aa0@kernel.org: (22 commits)
  selftests/coredump: add second PIDFD_INFO_COREDUMP_SIGNAL test
  selftests/coredump: add first PIDFD_INFO_COREDUMP_SIGNAL test
  selftests/coredump: ignore ENOSPC errors
  selftests/coredump: add debug logging to coredump socket protocol tests
  selftests/coredump: add debug logging to coredump socket tests
  selftests/coredump: add debug logging to test helpers
  selftests/coredump: handle edge-triggered epoll correctly
  selftests/coredump: fix userspace coredump client detection
  selftests/coredump: fix userspace client detection
  selftests/coredump: split out coredump socket tests
  selftests/coredump: split out common helpers
  selftests/pidfd: add second supported_mask test
  selftests/pidfd: add first supported_mask test
  selftests/pidfd: update pidfd header
  pidfs: expose coredump signal
  pidfs: drop struct pidfs_exit_info
  pidfs: prepare to drop exit_info pointer
  pidfd: add a new supported_mask field
  pidfs: add missing BUILD_BUG_ON() assert on struct pidfd_info
  pidfs: add missing PIDFD_INFO_SIZE_VER1
  ...

Link: https://patch.msgid.link/20251028-work-coredump-signal-v1-0-ca449b7b7aa0@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>