pidfs, coredump: add PIDFD_INFO_COREDUMP

Extend the PIDFD_INFO_COREDUMP ioctl() with the new PIDFD_INFO_COREDUMP
mask flag. This adds the @coredump_mask field to struct pidfd_info.

When a task coredumps the kernel will provide the following information
to userspace in @coredump_mask:

* PIDFD_COREDUMPED is raised if the task did actually coredump.
* PIDFD_COREDUMP_SKIP is raised if the task skipped coredumping (e.g.,
  undumpable).
* PIDFD_COREDUMP_USER is raised if this is a regular coredump and
  doesn't need special care by the coredump server.
* PIDFD_COREDUMP_ROOT is raised if the generated coredump should be
  treated as sensitive and the coredump server should restrict to the
  generated coredump to sufficiently privileged users.

The kernel guarantees that by the time the connection is made the all
PIDFD_INFO_COREDUMP info is available.

Link: https://lore.kernel.org/20250516-work-coredump-socket-v8-5-664f3caf2516@kernel.org


Acked-by: Luca Boccassi <luca.boccassi@gmail.com>
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Jann Horn <jannh@google.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>